The Farrelly brothers helped popularize the modern gross-out comedy in their early flicks, beginning with “Dumb and Dumber,” and they reached a sort of inspired lunacy in its immediate successors, “Kingpin” and “There’s Something About Mary.” But their movies–not only the ones they’ve written and directed themselves, but those they’ve sponsored, too–have gone rapidly downhill since those glory days. Not only has the gross-out factor curdled in lame efforts like “Me Myself & Irene” and “Say It Isn’t So,” but when they try to go a sweeter, kinder route (as in “Shallow Hal”), the effort seems to sap their energy. Take this picture, about conjoined twins. One can only imagine how rude and crude it would have been in the days of “Kingpin” and “Mary.” Now the tone is so mild, innocuous and inoffensive as to be positively boring. Think, for example, of all the scatological gags that the Farrellys of the ’90s would have included in such a project (just recall Jeff Daniels’s bathroom scene in “Dumber,” or the famous hair gel moment in “Mary”). Well, none of them are present here. There is one joke about urination, but it’s so incredibly tame it occurs in a Jay Leno “Tonight Show” bit (and the very fact that the overexposed Leno is employed here is a sign of the desperation at work), and an equally sedate one about masturbation, but that’s about it. It’s as if the filmmakers were more sensitive about portraying the physically and mentally challenged in a kindly light than interested in generating laughs. You might call the result “Bland and Blander”–the picture is sometimes mildly amusing but more often merely tedious, consisting of a series of tame variations on a theme that wasn’t all that inspired to begin with.

Of course, one might be relieved that the brothers aren’t trying to outdo their earlier gross-out stuff (“Irene” proved how hard that is) or that of their acolytes (“Say It Isn’t So” is the worst example) or their competitors. But that old exuberant vulgarity needs to be replaced by something more than the sort of tepid material on display in “Stuck On You.” The best part of the movie comes early on, when we’re introduced to conjoined twins Bo and Walt Tenor (Matt Damon and Greg Kinnear) in their native environment of Martha’s Vineyard, where they run a burger joint. Though even here the atmosphere is muted by the strain to be politically correct (a sequence in which a ruffian customer complains loudly about “freaks” clumsily makes the point), there are some good gags–especially when Walt, a thespian at heart, plays the title role in “Tru,” dragging his supportive but stage-fright afflicted brother across the boards with him. Maybe if the script had stayed at this local level it might have worked, but it takes a disastrous turn when the boys go to L.A. for Walt to follow his unlikely dream to get into movies and TV. The picture turns into a really lame soft-hearted satire of “the business,” with Cher doing the bitch diva bit as herself, hiring Walt for a terrible series she’s contractually committed to in order to sink the project, and Seymour Cassel showing up as a sleazy agent. Things get so bad that we’re supposed to find the program, called “Honey and the Beaze,” a hilarious takeoff, although what it spoofs–the dreadful private eye shows (“Barnaby Jones,” “Cannon,” etc.) of the seventies and eighties–are so passe that they’re hardly ripe for even the dimmest satire, and to be surprised when it turns out to be an unexpected ratings smash. And we’re meant to be amused by the sight of the over-the-hill agent sporting a monumentally hideous hairpiece or puttering around in one of those old folks’ mobile chairs. As for the guys, they’re naturally given some romantic material. Walt takes up with a none-too-bright but open-minded neighbor (Eva Mendes), while the nervous Bo woos the Chinese girl (Wen Yann Shin) he’s corresponded with over the net for years but never told he’s linked to his brother. That leads to lots of silly slapstick in which the duo try to hide the fact they’re conjoined while going out with her, punctuated with incidents in which insensitive types demonstrate their boorishness by teasing the boys. All the Hollywood stuff and relationship problems are good for no more than mild chuckles, but it’s nonetheless preferable to the increasingly sentimental turn the movie takes toward the close–and to the big musical production number at the end, which is meant to be show-stoppingly wild but falls flat, despite the fact that Meryl Streep makes a pointless and decidedly unfunny cameo in it. (Who thought that it would be a riot for Bonnie and Clyde to sing “Summertime” in the first place? “Springtime for Hitler” it’s not.)

As to the cast, Damon shows that while he can do drama (“Ripley”) and action (“The Bourne Identity”), he hasn’t much flair for comedy. (It doesn’t help that Bo changes radically from moment to moment, suddenly turning from a terrified, panic attack-prone fellow on stage to a willing celebrity and even script doctor, only to revert promptly to his earlier persona when the narrative demands it.) As if to compensate, Kinnear works desperately hard to inject some life into the tired material, but his combination of mugging and smugness (call it “smugging”) quickly grows exhausting. The veteran Cassel follows his lead by playing as broadly as he ever has (which is saying quie a lot); Mendes and Shin are basically decorative. In addition to the good-sport appearances of Cher and Streep, Griffin Dunne shows up as the long-suffering director of “Honey and the Beaze.” But he doesn’t suffer quite so much as we do watching him. The best brief spot actually comes from Frankie Muniz, and it lasts all of fifteen seconds. On the technical side the picture is mediocre; the widescreen lensing by Dan Mindel, as well as all the production credits, are at best functional. (The twinning prosthetics are especially poor.) In this they go along with the Farrellys’ directorial technique, which actually seems to be regressing; individual scenes here are haltingly staged and drag on much too long.

There was a time when the Farrellys were the new face of Hollywood comedy, but they’ve fallen behind the curve faster than Mel Brooks did. If their latest hasn’t reached the depths of “Spaceballs” or “Life Stinks,” it’s still a pretty lukewarm affair.