When Walt Tenor (Greg Kinnear) decides he wants to become an actor, he tries to convince his twin brother Bob (Matt Damon) -- his conjoined twin brother -- to move out to Hollywood with him by saying, "You could be my stunt double!"

Yes folks, "Stuck On You" is another cheeky comedy of good humor and questionable taste from the Farrelly Brothers ("Kingpin," "There's Something About Mary" and "Shallow Hal"), and yes, folks, they get a surprising amount of mileage out of jokes like that one -- rim-shot-quality punchlines given winkingly ironic sparkle by the wily writing-directing team's laughing-with-not-laughing-at sensibilities.

There's the scene in which Walt walks his shy sibling over to a pretty blonde in a bar, then takes over the seduction himself when Bob blows it -- and ends up bringing the girl home (Bob tries to ignore their moaning from the other side of a makeshift curtain). There's Walt's "one-man" stage show about Truman Capote, in which Bob tries to slouch as inconspicuously as possible behind Walt's back.

But always supportive of each other's ambitions, Bob agrees to leave behind their co-owned burger joint, where together they're the speediest fry cooks in Martha's Vineyard, in order to go with Walt to Hollywood, where the first friend they make is a dingy, implant-busty wannabe actress (Eva Mendez) whose familiarity with elective surgery leads her to jump to a conclusion that could only be reached in L.A.

"So, where did you get this done?" she asks of their connected torsos.

None of this sounds as funny in print, but the Farrellys have a gift for politically incorrect comedy that can turn mild snickers into out-loud laughs, and in "Stuck On You" they succeed more than they fail. But they also have a tendency to rely on good gags to prop up shopworn storytelling, so despite its weird premise, "Stuck On You" all too often moves in obvious directions.

While dragging his brother along on auditions proves bad luck for Walt's career, Bob tries many wacky ways to hide his condition from a girl he met on the internet (Wen Yann Shih) in a subplot that feels perfunctory, telling her that his brother is just "clingy." These one-joke premises get played over and over again.

But then along comes Cher (willfully mocking her public image as an uber-diva), who is trying to get out of her contract to star in a hokey DNA-detective drama on TV. She has final approval on the casting of a co-star, so when she meets Walt, she hatches a plan to freak out the studio suits and get the show cancelled. This also opens the movie up to poking a little fun at the entertainment industry by way of Seymour Cassel playing Walt's aged agent who is so out of touch that when the truth gets out about his client, he hollers, "Cronkite's gonna have a field day with this!"

Kinnear and Damon (whose lack of resemblance is the movie's big unspoken joke) have good chemistry and comedic synchronicity, which no doubt evolved out of being lashed together in costumes or prosthetics for the length of the shoot. One of the funniest scenes in the movie comes when the brothers are bullied in a bar and fight in perfect unison to give a band of troublemakers a back-to-back butt-kicking straight out of a kung-fu-movie.

Yet because "Stuck On You" is so long on premise, it seems that much shorter on the kind of belly-jiggling, cheek-sore japes that make a comedy memorable and have made the Farrellys famous. If you're itching for that brand of audaciously crass laughter, you're not going to find it here. But the movie does have a matinee-ticket's (or video rental's) worth of screwball amusement.