Will Ferrell plus Adam McKay usually equals comedy magic. They made Anchorman and Talladega Nights together, and movies don’t get a whole lot funnier than that. Step Brothers doesn’t come close to attaining the same comic altitude, however, and I think the problem is that Judd Apatow got factored into the equation.

On paper, the combination of creative influences might have looked like an inspired idea, but on the screen it proves an unfortunate case of worlds colliding. The styles that have brought McKay and Apatow to prominence don’t have much in common and don’t mesh particularly well.

Caught in the artistic crossfire are Ferrell and Talladega costar John C. Reilly. They play a couple of 40-ish arrested-development cases who live with their single parents and have never worked a day in their lives. Ferrell is Brennan. He lives with his mother, a successful businesswoman played by Mary Steenburgen. Reilly is Dale. He’s the spoiled spawn of a well-to-do doctor played by Richard Jenkins. When the mother and father meet, fall in love, and marry, life as the two freeloaders know it is over. Not only will they have to share their parents’ attention with someone else, they’ll also have to share a room.

The makers of the movie never quite get around to explaining how it happened that two accomplished professionals allowed their offspring to languish developmentally to such a degree. Nor do they account for the violent and surly dispositions of the overgrown brats. The viewer, it would appear, is simply expected to be content with the improbable spectacle of middle-aged comedians behaving like pampered 10-year-olds.

The first half of the movie has Brennan and Dale at each other’s throats. Reilly’s character, for example, warns Ferrell’s against going anywhere near his precious drum kit. Ferrell responds the way any self-respecting man-child would: He barges into the room where the skins are set up and proceeds to rub his “nutsack” all over them. As the film is produced by Apatow, there is, of course, an actual “nutsack” (presumably made of latex) protruding from Ferrell’s unzipped pants. Another movie milestone for the reigning king of comedy.

The picture’s second half sees the simple-minded siblings united by a common disdain for Brennan’s younger brother Derek (Adam Scott, whom you’ll eventually recognize as the delivery room assistant in Knocked Up). He’s everything they’re not — successful, toned, married and overflowing with self-confidence. When Dale punches him in the face, knocking him out of their tree house, Brennan realizes he and his new roomie aren’t so different after all, and there ensues a series of scenes in which they share their passions for velociraptors, nudie mags, Shark Week and Star Wars.

Step Brothers runs only 93 minutes, but it feels substantially longer than that. Too many gags grow old, and the picture runs out of gas for a spell in the second act. Reviewers are always talking about how certain comedies play like “Saturday Night Live” sketches padded and stretched to feature length. This one had me musing, conversely, that it might have made for a gutbusting five or 10 minutes of TV. Ferrell and Reilly have phenomenal chemistry and give the film everything they’ve got. The problem is that the picture’s creators just don’t give them a heck of a lot to work with, beyond the high-concept premise and a lot of gratuitously potty-mouthed dialogue.

There’s an everything-but-the-kitchen-sink quality to the humor here. A handful of scenes do work: The one where Brennan and Dale interview for a job tag-team style is an instant classic. The film offers a nonstop barrage of off-the-wall ideas, and occasionally one of them hits its target, as when Dale is forced to see a shrink and, asked for his life story, offers up the plot of Good Will Hunting.

These are exceptions, unfortunately. As a rule, Step Brothers doesn’t come close to succeeding. It’s not quite what we’ve come to think of as an Adam McKay film. And it’s far too angry and too much of a cartoon to meet our expectations for a Judd Apatow creation. More than anything, it brings to mind the kind of man-child comedy that was at one time the purview of Adam Sandler. There’s a reason he stopped making those. I’m not sure there’s a good reason why Ferrell and Reilly would start.