Compared to the pathetic protagonists in "Step Brothers," Steve Carell's 40-year-old virgin is a ladies' man and Matthew McConaughey in "Failure to Launch" is a go-getter.

Brennan (Will Ferrell) and Dale (John C. Reilly), middle-agers who act like they're in middle school, are forced to bunk together because they're still living with their respective parents, who meet and marry.

Funnier roomies haven't been spotted since Felix and Oscar. The guys don't like each other, and each focuses his tiny brain on making misery for the other. The process involves name-calling, violent threats and the defiling of prized possessions.

Back together are the core filmmakers who made "Talladega Nights: The Ballad of Ricky Bobby" (2006): the two stars along with producer Judd Apatow and director-writer Adam McKay. Both films boast the goofiness of a bunch of pals chuckling about their childish ideas for skits, rushing out to perform them on a webcam and posting them on YouTube before the buzz wears off.

Nothing in "Talladega Nights" or especially the cheerfully vulgar "Step Brothers" feels focus-grouped, cross-referenced or refined. Much of the movie is Ferrell and Reilly cussing each other out with the mentality of two 12-year-olds who sit in the back of class composing armpit symphonies.

Ferrell is an astounding childish talent, but his films tend to coast after a blisteringly funny first half-hour that drains all the juice out of flimsy premises. "Step Brothers" also flattens after 30 minutes but manages to string together more than enough punch lines to keep your eyes on the screen instead of your watch or the exit signs. Reilly, who floats effortlessly back and forth from dramas to comedies, is Ferrell's kindred spirit of arrested development.

What makes the pair of losers such a winning combination is just how dumb each character is willing to be to belittle the other. Sometimes, idiocy approaches genius – for instance when one snaps a nasty joke and the other responds by saying it was funny the first time he heard it, when he fell off his dinosaur.

As entertaining as the interplay between the leads is, the two stalwart straight-man turns by Mary Steenburgen and Richard Jenkins, who play the parents, magnify the hilarity.

Both add such frustrated naturalism to the roles that you can believe these are the types of pushovers who might let their offspring mooch off them for decades. There are notes of beleaguered duty that underlie the parents' token discipline and flustered reactions, as if the fact that their boys still live with them is a testament to their failures as parents.

A stinging counterpoint to the leads' aimlessness is Adam Scott, as Dale's hypercompetitive braggart of a younger brother who rubs everyone's face in his success. Every other line of his is a "did he really just say that?" rib-rattler.

In a lesser comedy, Scott would have stolen the film. But the Ferrell-Reilly brotherhood can't be breached.