It’s always a weird thrill for viewers when characters from popular TV shows spend time in each other’s universe. It’s like, what’s Mork from Ork doing in the Cunningham’s living room on “Happy Days” asking Fonzie for dating tips? Before you can say, “Holy triple crossover,” the premise only gets funnier when Fonzie decides to have Mork try out his moves on Fonzie’s pal, Laverne, of “Laverne & Shirley.”

That programming move from the late 1970s was an example of ABC multiplatforming magic before the term even existed.

Crossovers usually happen to shine the spotlight of a hit show on a new one. But sometimes they arise out of one star’s sheer admiration of another, as when the stars of “Absolutely Fabulous” found themselves getting drunk in an episode of “Roseanne” in 1996. It was the show’s final season and Roseanne Barr was eager to produce a US version of the outrageous British series, but the closest she ever got — since the behavior of Patsy and Edina would have never gotten past prim network censors — was to have them on her show.

This week the cast of CBS procedurals “Hawaii Five-O” and “NCIS: Los Angeles” visit each other’s states to solve a crime together. Strange as it may seem for these unrelated shows to hook up, it’s not that far-fetched.

“The task force that works out of Hawaii is contemporary with the special unit that works out of LA,” says “NCIS: LA” creator Shane Brennan. “Post 9/11, there’s been a lot more cooperation and multi-jurisdictional task forces working together, so we extended that and it works in this [fictional] world.”

The investigation, which concerns someone who’s threatening to disperse a deadly virus, begins Monday night on “Five-0,” in Hawaii, before jumping to the mainland on Tuesday, where it wraps up on “NCIS: LA.”

An uncovered personal connection to the case sends “NCIS: LA” characters G. Callen (Chris O’Donnell) and Sam Hanna (LL Cool J) to Hawaii. Then, when the suspect ditches the islands for LA, they hop the red-eye home, bringing Danny Williams (Scott Caan) and Chin Ho Kelly (Daniel Dae Kim) of “Five-0” with them.

Brennan says it makes sense to split up the story up over two episodes. “They jump on the plane on Monday night and fly to LA on Tuesday, so it does have that sense of being in real-time,” he says.

While the timing was a no-brainer for each show’s show’s writing staff, capturing the established attitude and cadence of the guest stars was more complicated.

Both audiences have to say, “‘Yeah, that’s our guys!’ ” Brennan says. “They can’t walk in [to the other series] and not be the characters that fans have come to know.”

To work around that, the two series asked their writers to do a little crossover of their own — “riding shotgun” and getting writing credits on each other’s series.

“To keep it true to those characters [who crossed over],” Brennan adds, the two shows also “broke the stories, outlined and worked on the scripts together.”

The only thing that nearly got lost in translation between the shows was wardrobe.

“At one point, Callen and Sam were going to be [wearing] Hawaiian shirts,” Brennan recalls with a chuckle. “I said, ‘No, I don’t think that’s going to work for our audience.’ ”

While the crossover could turn into a ratings bonanza — they’re two of CBS’ most-popular dramas — Brennan says that the shows did it purely to entertain fans.

“They’re going to sit back and say, ‘That was fun, that’s was different, that was great to watch,” he says. “If our numbers go up and ‘Five-0’s’ go up, that’s a great thing, too.”

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