CSI + 2 1/2 = ?

That Chuck Lorre should have a funny idea should come as no surprise. He's the co-creator and executive producer of both "Two and a Half Men" and "Big Bang Theory."

But nothing on those shows is any wackier than the "Two and a Half Men" writers writing an episode of "CSI" and vice-versa.

"I mentioned it as something ridiculous ... about a year ago," Lorre said. "It went viral from there."

And the results are on the air this week. CSI executive producers Carol Mendelsohn and Naran Shankar wrote tonight's "Two and a Half Men" (8 p.m., Ch. 2); Lorre and "Men" co-creator and executive producer Lee Aronsohn wrote Thursday's "CSI" (8 p.m., Ch. 2).

Mendelsohn said that when Lorre called her with the idea, she "went into Naran's office and he said, 'What a nut!"'

"I've been called much worse," Lorre said.

Before long, executives at both production companies and the network were enthusiastically on board. Much to Lorre's surprise.

"The fact that it's taken hold and we're having this conversation now defies every sense of reality that I have," he said. "It's mind-boggling that we actually did this."

The shows' stars were taken aback. Aronsohn said Charlie Sheen's first reaction was, "Are you serious?" Shankar said William Petersen's first reaction was, "Are you crazy?"

"And, 'I'm not crossing over,"' Mendelsohn added.

(A few of the series' stars make cameo appearances on the other show, but this is a writers' crossover.)

But in tonight's episode of "Men," there's a suspicious death at Charlie's (Charlie Sheen) house. There are suspects, interrogations, an autopsy — the kind of stuff you see on "CSI" every week.

"The biggest challenge for us was doing a comedy with a murder in it. I mean, generally, our stories are a little lighter," Lorre said.

"Our characters usually don't have to react to corpses," Aronsohn added.

On Thursday's "CSI," a sitcom diva is murdered in Las Vegas, and the investigation takes Grissom (Petersen) to Los Angles."

The writers operated like freelancers — they brought in an outline and "then the writing staff takes it apart and puts it back together again," Lorre said.

"They gave us the forensics and the plotting of the crimes, and we kind of put in our little ha-ha's," Aronsohn said.