What do you do if the best player in the American League is really two players platooned at one position? Who goes to the All-Star Game? How do you vote for such an entity for the most valuable player award?

In baseball's history, such a problem has never come up. That is, until now.

Baltimore's two left fielders, Gary Roenicke and John Lowenstein, are at it again, producing more offense between them than any other team in the AL gets out of any one position on the field.

This evening it was Roenicke who did his imitation of half a Hall of Famer. Twice against Shane Rawley he hit two-run homers over the left field wall, providing all the explosives the Orioles needed to thump the New York Yankees, 5-2, before 31,173.

The Orioles' left fielders, who had 41 homers and 123 RBI last year, are now harming pitchers at an even faster pace. After 66 games, they have 15 homers and 60 RBI. This night's effort gave Roenicke 11 homers and 31 RBI in 121 at bats--the sort of statistics customarily associated with Babe Ruth.

His first homer, in the fourth inning, was a legitimately titanic shot, landing 30 rows up in the 40-row-high bleachers in the corner. The Rhino's second blow for platoon power was a game-winning liner in the eighth that disappeared in a great hurry over the fence in left-center.

That crisp bit of execution, done with the quickest and most compact of strokes, broke a 2-2 tie. It also made Yankees Manager Billy Martin--fresh from his latest brushes with common indecency--wonder why he'd left a tiring starter in the game when Goose Gossage was heated to well done in the bullpen.

Lowenstein, who spent the evening on the bench honing his one-liners, has eight homers and 32 RBI in 140 at bats. A fan of the movie "The Alien," he refers to himself and Roenicke as "The Entity."

"Let Gary go to the All-Star Game to represent the Entity," he said. "I already have plans to go to the Smithsonian."

This victory, giving the first-place Orioles a 16th in 23 games, was a clean and simple affair. Starter Storm Davis was excellent for 7 2/3 shutout innings, striking out eight and exhibiting the best changeups of his brief career.

The sight of Davis looping his rainbow curve over for strikes, spotting his fine fast ball on the outside corner like a Jim Palmer and going to changeups in audacious spots--for instance, a 3-2 lollypop that turned Roy Smalley inside out--gave a hint of the hurler the youngest player in the AL may become. However, one Yankee veteran, Graig Nettles, was too savvy for the kid.

With a man on in the eighth, Nettles dove into an outside fast ball and drove it over the left-center fence for a game-tying homer that took a victory from Davis. "Our pitchers call Nettles 'Scuba' because he's always diving across the plate like that," said Ray Miller, the Baltimore pitching coach.

Thereafter, the Orioles put their hill chores in the left hand of Tippy Martinez, who has made a career out of proving to the Yankees that they never should have traded him seven years ago. Martinez, now 5-3 after getting the final four outs, has not allowed the Yankees a run in his last 10 appearances against them, 15 2/3 innings. In 44 games in his career, his ERA against the Bronx contingent is 2.39--or about a full run lower than against everybody else.

When Davis and Martinez needed defensive help, they always got it--usually from (who else?) the left fielder. True, second baseman Rich Dauer, who doubled in the final run against Gossage, made two sparkling diving stops. But it was the languidly efficient Roenicke who stole the glove show, too. He got a great jump on Dave Winfield's liner in the fifth and shagged the ball at a gallop to prevent an RBI double. In the seventh, he made a sliding, grass-high catch of Smalley's drive to end the inning with a man on.

This victory merely continued the Orioles' longstanding and somewhat inexplicable domination of the Yankees.

Since 1964, they have a 190-120 (.615) mark against the Yankees and have won 15 season series. In recent years, they have done even better, going 26-10 since late April of 1980. New York has a 4-17 record in Memorial Stadium in the 1980s, now having lost 10 straight.

"I don't know how to explain it, but it sure is fun," said Baltimore General Manager Hank Peters of the two left fielders who first blossomed in 1979 for 36 homers and 96 RBI in 573 at bats. "I know they couldn't get the most valuable player award, but after what they've done . . ."

Peters and Manager Joe Alotbelli agree that Roenicke, 28, will inherit a full-time job when Lowenstein, 36, starts to show his age. For now Altobelli says, "How the hell do I change something that's working like that? Every player wants to play every day . . . and I think you'll see Gary get that chance one day . . . but, right now, maybe the rest does them both more good than harm. Sometimes players can't see the forest for the trees. They think of 'me' not 'we.' But John and Gary are exactly the opposite."

Orioles catcher Joe Nolan was put on the 15-day disabled list with a chip fracture of the big toe on his left foot. He was hit by a pitch Sunday. He will be replaced by John Stefero, left-handed-hitting catcher batting .312 with 16 homer in 60 games for the Class AA Charlotte affiliate