A Farrelly brothers movie with a PG-13 rating should be a red flag to audiences.

The filmmaking team of Bobby and Peter Farrelly epitomized the gross-out humor of the past decade with standouts such as “There’s Something About Mary,” which ushered in a new critical and commercial appreciation for the cinematically vulgar.

Only recently have they tried to sentimentalize their material, starting with 2001’s “Shallow Hal.” Now with “Stuck On You,” they’ve taken a potentially outrageous idea — a comedy about conjoined twins — and turned it into a frequently funny but unmemorable time-passer.

The film is distinguished by some good performances (Greg Kinnear), some run-of-the-mill ones (Matt Damon) and a few excellent extended cameos from big-name stars such as Meryl Streep and Cher. If only the project was a little edgier, a tad more over the top … in other words, if only it was more “R.”

Kinnear and Damon play Walt and Bob Tenor, a pair of “Siamese twins” who own a popular diner in Martha’s Vineyard. Although they share a liver — which has prevented surgical separation — they lead fairly diverse lifestyles from each other.

Walt (Kinnear) is the outgoing one. He is far more confident with the ladies than the shy Bob. Plus, he has found success as a theatrical actor, where his friends and co-workers pack the auditorium to watch his yearly “one-man show.” (His latest is called “Tru,” based on the life of Truman Capote. What about Bob? He dresses from head to toe in black and tries to stay out of the way. It doesn’t help matters, though, that he suffers from intense stage fright.)

The local productions are just not enough for Walt, however, who finally convinces his twin to relocate to Los Angeles in order to pursue the dream of becoming a full-time actor. Fortunately, Bob is lured at the chance to meet May (Wen Yann Shih), a Chinese immigrant who now lives in L.A. with whom he’s been cultivating an Internet romance — even though he never got around to telling her about his physical situation.

This leads to several awkward first dates where the brothers must find different excuses for why they are always standing next to each other. (“He can be a little clingy,” Bob apologizes.)

Matt Damon, left, and Greg Kinnear play conjoined twins who relocate to Los Angeles so one can pursue an acting career in Stuck

“Stuck On You” is pretty much a slice-of-life tale — albeit a quirky one — concerning the daily routines of its lead characters. The humor of the Tenors’ predicament doesn’t come from how awkward things are for them, but how EASY things are.

They are completely comfortable with their own bodies, as are the townspeople of the insular community who befriend and respect them. And in most instances they excel at whatever they do, whether functioning as a formidable goalie on their amateur hockey team or when speed-cooking orders at their burger joint.

A little of this stuff goes a long way, and the picture only really kicks into gear when the Tenors move out West and Walt lands a gig on a network TV series. Here, the Farrellys are able to skewer Hollywood more than just rely on the next dual sight gag.

Particularly game is Cher, who stars as herself while playing Walt’s co-star in a ridiculous new crime show. The filmmakers are so mean to Cher, depicting her as a superficial, vindictive prima donna, that it’s a real question if the diva even understood to what extent she’s being mocked.

What works best about “Stuck On You” is the choice to give Bob and Walt such different personalities. Somehow it’s more amusing that they dress completely dissimilar than if they had eccentric matching outfits. They don’t even look like guys who would hang out together, let alone be attached at the hip. (Is it a coincidence they have the same basic haircuts as the title characters from the Farrelly’s “Dumb and Dumber?”)

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The Tenors are pretty interesting, especially the more ambitious Walt, although the movie has trouble padding their story out to two hours.

All this leads to a way-too-long musical finale that falls entirely flat. The Farrellys seem to be setting the stage for a misguided production with “Springtime for Hitler”-like awfulness. But all they manage to do is make a rather sub-par homage to “Chicago.”

This is indicative of why “Stuck On You” needed that extra helping of raunchiness. Toning down the humor doesn’t make the film more endearing, just more forgettable.

Matt Damon, left, and Greg Kinnear portray conjoined twins who become part of a new TV series starring Cher in the Farrelly brothers' comedy, Stuck