Stuck on You marks the third time since ''There's Something About Mary'' that the Farrelly brothers have cowritten and directed a comedy in the form of a freakish, screwball case study. Only now they've finally gotten it right. ''Shallow Hal'' made almost no sense (when Jack Black's callow skirt chaser looked at a fat woman and saw a superbabe, how, exactly, had he outgrown his addiction to superbabes?), and ''Me, Myself & Irene'' got stuck in the manic gears of Jim Carrey's babbly virtuosity. In ''Stuck on You,'' however, the Farrellys invite the audience to think, feel, and giggle their way right into the lives of Bob and Walt Tenor (Matt Damon and Greg Kinnear), a pair of brothers united by a pulpy wedge of skin (both have full, working sets of arms and legs). The film pushes these conjoined twins well past the point of absurdity, mocking and celebrating the droll logic of their anatomical-emotional predicament until it looks like the most natural state in the world.
Sporting dumb and dumber haircuts, Damon, as the earnest, nerdish Bob, who's given to panic attacks, and Kinnear, as his (slightly) cooler brother, shamble about with a graceful hiccup of a walk. Working as short-order cooks on their home turf of Martha's Vineyard, they dish up burgers like a two-man Rube Goldberg machine, and we also get to see the loopy spectacle of one, or both, doing the following things: jumping rope, pitching baseball, getting laid, playing Truman Capote on stage (you heard me). The Farrellys, though, are up to more than serving up logistical gags of deeply questionable taste. Spurred by Walt's dream of an acting career, the brothers take off for L.A., where Walt ends up starring opposite Cher (playing herself with good-sport glee) in the TV series ''Honey and the Beaze.'' ''Stuck on You'' has a fractured fairy-tale charm, even if it isn't a nonstop laugh riot. It's really a love story in which Damon and Kinnear, playing what may be the closest buddies in buddy-movie history, bring off the resonant feat of making Bob and Walt genially devoted, slightly mad innocents who have to tear themselves apart to put themselves together.