Audi Field, Washington’s new soccer stadium, can be seen from Nationals Park. Some fans attended both games when the two teams played back-to-back on a Sunday in August. (Katherine Frey/The Washington Post)

Rob Castro was sipping beer on the upper concourse of Nationals Park on an August Sunday — a sixth-inning break from a game that would last two hours and 40 minutes — when he spotted something new two blocks away: Audi Field, the just-opened home of D.C. United, getting ready for a night match of its own.

It was the first time the two teams played at home on the same day, and Castro decided to pull a split-sport doubleheader. He took out his phone, booked $44 seats on Ticketmaster and prepared to switch from hot dogs (and beer) to pupusas (and beer).

Castro, an AT&T retail manager from Northern Virginia, loves “America’s pastime” but lives for “the world’s game,” a passion inherited from his Salvadoran-born father. “They’re both so different,” Castro said.

And yet so close. In Washington, baseball and soccer are now parallel universes served by the same Metro station. On days when both teams play, as they will again Sunday, traveling from one to the other is a short walk between distant planets.

The diamond, in Southeast D.C., and the pitch, in Southwest, are just a couple of Bryce Harper homers away from each other, and both are part of the dizzying gentrification of one of the city’s grittiest areas. Both draw fans by the thousands to drink rosé and eat fusion tacos in the footprint of old scrapyards and strip clubs, even as cranes tug the skyline and housing prices ever higher.

Pupusas are a crowd favorite at D.C. United games. (Katherine Frey/The Washington Post)

Peanuts are found everywhere at Washington Nationals games. (Katherine Frey/The Washington Post)

D.C. United fan Sidny Oyola reacts as the team almost scores against the New England Revolution. (Katherine Frey/The Washington Post)

A toddler zooms out of the Brew House at Nationals Park. The Northern Virginia boy attended his first game as a newborn. (Katherine Frey/The Washington Post)

But Audi Field is making the boom wider, younger and more diverse. The fan base pouring from the Green Line to watch soccer is 44 percent white, according to team statistics, with Hispanics and African Americans making up 25 and 23 percent respectively. By contrast, nearly 65 percent of Nats attendees are white, according to a team spokesman, while just over 13 percent are black and 9 percent Latino. D.C. United claims it draws the most millennial crowd of any D.C. sports team, though the Nationals boast they draw a half million fans under 30 every year.

Both teams attract affluent crowds — people with incomes that top $100,000. The cheapest seats at Nationals Park are about $11 (excepting a few $5 day-of-game walk-up tickets) but can spike as high as $345, according to prices listed for upcoming games. Audi Field’s prices, depending on the matchup, range from about $22 to $275.

With those demographics at play, the culture, cuisine and crazy factor inside the two stadiums are as different as curveballs and corner kicks.

“In baseball, you only scream when they score,” shouted Alex Whelton, a hoarse 9-year-old bouncing that night with his father and twin brother in the never-quiet section behind Audi’s north goal. He had bellowed through the first 45 minutes of United’s match against the New England Revolution; the green plugs sticking from his ears were given to him by the shirtless bass-drum banger a few seats away.

Three D.C. United groups, the District Ultras, La Barra Brava and the Screaming Eagles, chant, cheer, yell and scream throughout the match accompanied by drums. (Katherine Frey/The Washington Post)

There are (marginally) quieter seats nearby, but the Wheltons are part of the long tradition of never-sit soccer fans who want to feel the game as much as watch it. The pulse is supplied by three team-sanctioned groups of supporters — the Screaming Eagles, La Barra Brava and District Ultras — that fill one end of the stadium with waving banners, ceaseless chants and fire-marshal-approved colored smoke.

This ritualized mayhem takes the place of the organ riffs, pre­recorded cheer prompts and batter walk-up music that make Major League Baseball such a produced experience. Before the game, Audi’s speakers pulsed with reggaeton. But after kickoff, the official team presence was limited to the official team, and they played to a fan-created soundtrack.

“It is a little bit like the energy of Mardi Gras in New Orleans,” said Sean Whelton, the twins’ indulgent father and an associate professor of medicine at Georgetown. His family gravitated to the frenziest sections during games at RFK Stadium, where United played for 22 years until the $400  million Audi Field opened in July next to a riverside Pepco substation.

Audi’s 20,000 seats, about half the capacity of Nationals Park, are easier to fill than RFK, where the team rarely needed to open the upper deck. It’s a more intimate setting, saturated with sound.

Washington Nationals center fielder Michael A. Taylor, third from right, attended a D.C. United game hours after the Nationals lost to the Miami Marlins. (Katherine Frey/The Washington Post)

“There is lots of energy, and it’s very exciting,” said Michael A. Taylor, who started his day at Nationals Park as a Nats outfielder and finished it at Audi, standing unrecognized in a line for an arepa. “I’m a big fan, but I don’t get many opportunities to see them because of our games.”

He wasn’t the only one at the night game after wrapping up a baseball day job. Jawaan Samuel, 22, hawked beer at both parks and reported two differences: The stairs are steeper at Audi, and the concentrated nature of a soccer game extends to its drinking patterns. United fans, who can be in and out in a couple of hours, clamor for his Tecates as soon they walk through the gates. At Nationals Park, they stretch it out.

“I don’t even go out at a baseball game until they sing the anthem,” Samuel said, taking a break after a United goal had created a shower of spilled beer followed by a lucrative rush for replacements. The degree of beer saturation in Audi’s air (brewmidity?) definitely spikes when something good happens on the field.

“It’s been a long time since anyone has spilled a beer on me for a home run,” noted Michael Rudolph, a Nats devotee at his first United game. He had been spontaneously invited to the match during the baseball game by a United supporter with a spare ticket. Rudolph enjoyed the outing but remained unpersuaded.

“Baseball is a thinking man’s sport.” he declared. “Soccer is just okay.”

There are plenty of crossover fans, of course, code switchers fluent in both “offside” and “infield fly.” They toggle between the parts of the brain that engage for two games that progress, respectively, like a picnic and a mosh pit.

The Tipton family from Takoma Park, Md., divided their day between the two games. By the third inning of the Nats’ series closer against the Miami Marlins, they were deep in the baseball brain waves, their focus on the field rising and falling with a game pace that averages around four minutes between balls in play.

Sean Tipton, a nonprofit executive in a Nats cap and a Wayne Rooney United jersey, sipped a 25-ounce can of Michelob as right fielder Adam Eaton stepped out of the batter’s box yet again, the organ music filling the lull with a few quick notes of polka. Linda Tipton, a psychologist, was deep into vacation planning with her son’s girlfriend, Emmy Kennedy. Her head snapped up when a bat crack alerted her to Eaton’s run-scoring double.

All were attuned to baseball as an experience of pauses punctuated by jolts.

“You just know that if it’s two outs with nobody on, things are going to be quiet for a while,” Sean Tipton said. But he was soon yelling for Teddy Roosevelt to get his bigheaded bulk moving for the fourth-inning Presidents Race. (Kennedy scooped up the $4 pool when Jefferson took it at the tape.)

Families queue up to get into the kids’ fun zone during a Washington Nationals game against the Miami Marlins. (Katherine Frey/The Washington Post)

Nine innings — at a minimum — with no time limit allows for plenty of wandering by mind and body alike. If the concession stands at Audi Field are all but deserted during the 90 minutes of official play (pupusas, $9; half-smokes, $9; José Andrés jamón y queso, $15), Nationals Park’s are seldom without a line (corn dog, $7; half-smokes, $10; pollo asado nachos, $13). Out-of-seat fans still follow the leisurely march to 27 outs on monitors or the radio play-by-play that fills the concourses and restrooms. Some don’t even do that.

“There is nothing more American than baseball,” said Doug McSwain, who had no idea of the score as he stood happily with buddies around a table covered in peanut shells and tallboy IPAs.

With 162 games in a season, there is no single moment — or game — that can’t be missed for some other stadium diversion. The line for the toddlers’ playground was 20 families long by the eighth inning (of, granted, a blowout loss for the Nationals).

The unhurried pace is just what many fans love most, including those who socialize and those who track each pitch.

“If they’re not going to give him an error, I am,” muttered Emory Riley, 56, marking an “E” after Harper’s name after the center fielder dropped a catchable flyball in the seventh. From his center field perch, he noted every at-bat in his spiral-bound Bob Carpenter Scorebook, as he does at every Sunday home game. Baseball as bookkeeping gives Riley a permanent record of a quiet pleasure.

“I was here when Max [Scherzer] got 20 strikeouts,” he said, flipping through the log that he sometimes peruses with his grandson.

Soccer, though? “Never my thing,” said the medical courier who grew up in Chevy Chase but now lives two hours away in Pennsylvania. “That’s a lot of running around for a score of 1-to-nothing.”

It was 2-to-nothing Sunday night, thank you very much, United’s fifth win out of six games in their new home. But that was enough for the sellout polyglot crowd.

D.C. United fan Mike Critchlow cheers on the team after they scored against the New England Revolution. (Katherine Frey/The Washington Post)

In a stadium named for a German sports car, fans shouted in Spanish, cheered for a Jamaican defender and clapped to an ancient Icelandic war cadence.

United’s new British superstar, Wayne Rooney, was welcomed to Washington with a chant of “Wayne the Swamp! Wayne the Swamp!” from the Screaming Eagles group.

Even the swearing had an international flavor.

“You wanker!” screamed Sean Tipton, then 10 hours into his sports-a-thon, when a New England player was ejected. (The soccer-crazy Kentucky native, 54, used to name his neighborhood pickup teams the Wankers and the Bollocks.)

Rob Castro, too, was more energized than exhausted after his very D.C. day. The Nats had lost, United had won, but both had left him feeling good about his city and its embrace of sports — and people — both new and old.

“I’ve got to get my father out here,” he said.