Just plopping down on the couch to watch the big game seems so slow, so lonely, so . . . 2010 to Lenny Beckerman. These days, when Beckerman watches his beloved New York Giants, as he will during Sunday’s Super Bowl, he doesn’t just watch them.

He keeps one eye on the set and another on his iPhone. Through Twitter, Beckerman curates a running stream of comments about the action from the 1,100 people he follows, re-tweeting the smartest quips to his followers.

“It’s like college when you had a bunch of buddies over for the game and everyone got rowdy,” said Beckerman, 38, a music-video and film producer in Los Angeles. “This is like watching the game with all these people.”

About 45 percent of Americans who own tablets and smartphones are like Beckerman, watching television while watching something else at the same time, according to a forthcoming study by Nielsen. About 36 percent of smartphone owners engage in the same behavior, the company said. The behavior is most common among teenagers (with about 53 percent of teen tablet owners doing it daily), but about 38 percent of tablet owners older than 55 do so, as well, said Don Kellogg, Nielsen’s director of telecom research.

The trend has spawned a sub-industry of apps designed to bring TV shows and computer devices together. Advertisers, meanwhile, are scrambling to keep up with the split attention of their would-be customers. Sunday’s game could be the first “second screen” Super Bowl, with a number of advertisers seeking not just the attention of TV viewers but “engagement” with them online. Coca-Cola, for example, will provide an online stream of its animated polar bears — one in a Giants’ blue scarf and the other in Patriots’ red — “reacting” to plays in real time, along with a Twitter feed in which the “bears” comment and answer questions.

Double-screen multitasking isn’t just a technological change; it suggests a revolution in social manners, too. Many of those tapping away on screens Sunday will be at parties or family gatherings. Which means that even on occasions designed for social interaction, there’s likely to be less face-to-face social interaction, though, as Beckerman noted, watching two screens facilitates plenty of give and take among people who are electronically connected.

“We always seem to be more interested in the person who isn’t there,” said Jeffrey Cole, director of the Center for the Digital Future at the University of Southern California’s Annenberg School for Communication and Journalism. Interrupting a conversation with another person in your presence to check your smartphone or laptop has become commonly accepted behavior, he said.

Two-screen behavior has its positive side. Cole recalls that he was out of the country for the past two Academy Awards shows but still “watched” the telecasts with his wife, who was at home in Los Angeles. Via tweets and texts, they shared their immediate reactions about the winners, chortled over amusing lines by the presenters and snarked on the fashions. Cole describes it as “watching together while not together.”

Live events — such as the Super Bowl, political debates and the State of the Union address — might offer the best opportunity for such instant exchanges. Most people watch those programs as they are broadcast, unlike regularly scheduled TV series, which many viewers record and watch at different times. Cole theorizes that the ratings for a number of live TV events over the past year have been pushed up by “co-viewing.”

TV networks have spent the past year devising ways to capi­tal­ize on the phenomenon. The stars of “Glee” might have been the first to hold live Twitter chats with fans during broadcasts. Many programs now do the same, posting Twitter search terms, known as hashtags, on the screen so viewers know how to comment.

Smartphone TV apps — from companies such as GetGlue, Miso and WiO — enable viewers to chat about shows in progress, answer trivia questions, respond to polls about plot developments and see behind-the-scenes footage. Several apps flash statistics during NFL games and serve up team news and tweets.

The biggest second-screen event in history might be Sunday’s game, with more than 100 million people expected to tune in to the broadcast on NBC, and an untold number following along on a second device.

That has prompted Go­Daddy.com, a domain-name registration site, to become the first Super Bowl advertiser to feature a QR scanning code in its Super Bowl TV ad. When scanned by holding a smartphone up to the screen, the code sends the user to the company’s Web site for more video of spokeswoman Danica Patrick.

Coke’s two TV commercials — one a one-minute extravaganza featuring a fumble-fingered bear — are designed to send people to Coke-related Web content that is “integrated” with each play of the game. The “Patriots” bear, for example, will celebrate when his team scores a touchdown or makes a big play; the “Giants” bear will look disappointed, and vice versa.

“It amazes us how teenagers can listen to music, watch TV, look at Facebook and text their friends all at the same time,” said Pio Schunker, Coke’s creative director for North America. “The question for us is, how do we reach someone whose attention is so divided? If that’s the reality, you have to make your [marketing] so entertaining and engaging on multiple fronts.”

The first and second screens might become one again soon. The new generation of “smart” TVs combines on one screen all the functions of a television and a computer, including access to popular Web sites and apps such as YouTube, Facebook and Twitter. Such televisions enable a viewer to watch “Dancing With the Stars” while simultaneously checking e-mail or sending tweets.

Someday soon, said USC’s Cole, you might be able to “invite” people to share their actual physical reactions to a program in a window of your TV screen, laughing, crying or cheering together as a show is broadcast. Which means no one will need to be prompted in the old-fashioned way. “Ultimately, this is the end of the laugh track,” he said.