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Fans of "The Flash" and "Supergirl" have been hoping for a crossover between the two series since before the latter premiered in October.

Greg Berlanti, the executive producer of both shows, has been thinking about it for longer than that — since he and his team were first working to bring "Supergirl" to television.

"Sure," he said. "What comic-book geek wouldn't love to see that?"

And Berlanti is a self-professed comic-book geek.

"We'd like to do it eventually," he said, shortly before "Supergirl" premiered. "Not right away, because we want to establish Supergirl and all the characters on that show. But eventually.

"It's not our decision, though. There are a lot of people involved."

Not just a lot of people, but people at different networks. "The Flash" airs on The CW; "Supergirl" on CBS.

It's no big deal when characters from NBC's "Chicago Med," "Chicago P.D.," "Chicago Fire" and "Law & Order: SVU" cross over from one show to another. Same thing with the three "NCIS" series on CBS.

But crossovers between networks are rare. CBS and NBC are competitors, so you're not going to see an "NCIS"-"Chicago Fire" crossover.

Such two-network events have happened occasionally, however. Back when David E. Kelly was among TV's most successful and powerful producers, he got Fox and ABC to agree to an "Ally McBeal"-"The Practice" crossover, and later a "Boston Public"-"The Practice" crossover.

And Det. John Munch (Richard Belzer), a character who originated on NBC's "Homicide: Life on the Street" and later moved to NBC's "Law & Order: SVU," crossed over onto Fox's series "Arrested Development" and "The X-Files," HBO's series "The Wire" and UPN's "The Beat."

There are other examples, but they're rare exceptions.

Yes, the title character of NBC's "Constantine" appeared in an episode of The CW's "Arrow," but not until after NBC had canceled "Constantine."

The multinetwork thing is a problem for "Supergirl" and "The Flash," but not that big a problem. The shows come from the same production company, Warner Bros., and from the same producers.

CBS manages and co-owns The CW, partnered with Warner Bros. Although it doesn't appear to be a coincidence that Barry Allen (Grant Gustin) is coming to "Supergirl"; Kara Danvers (Melissa Benoist) isn't going to "The Flash." (The episode airs Monday, 7 p.m., Ch. 2.)

CBS executives were cool to the idea of crossovers when questioned about the possibility, although the ratings of "Supergirl" — which have declined considerably since the show's debut — may have caused them to reconsider.

On The CW side of this, it's no big deal. The Flash first appeared on "Arrow," and there have been multiple crossovers since he was spun off into his own show in 2014. Characters from both series were spun off into a third series, "DC's Legends of Tomorrow."

Multinetworks is one thing; the DC multiverse is another. All three CW series exist in the same universe; "Supergirl" exists in an alternate universe.

This isn't coming out of the blue for "Flash" fans, where the multiverse has been a big part of the narrative since the series began. Barry has battled both the Reverse Flash and Zoom from Earth 2, hopping from one universe to another.

In Monday's episode of "Supergirl," he accidentally ends up on Kara's Earth. The two battle Silver Banshee and Livewire before Kara helps Barry get back home.

Technically, this isn't the first crossover between the two shows. In an episode of "The Flash," as Barry was transporting from Earth 1 to Earth 2, one of the images that flashed on the screen was — you guessed it! — Supergirl.

And this isn't the first time Gustin and Benoist have worked on the same show. They were both on "Glee" — Gustin as Sebastian Smythe; Benoist as Marley Rose — although their characters barely interacted.

Whether the "Flash"- "Supergirl" crossover is the first of many remains to be seen, but it's definitely on the minds of the shows' producers and stars.

Gustin told Entertainment Weekly that crossovers between "Supergirl" and all three CW series — "The Flash," "Arrow" and "Legends of Tomorrow" — are "in the cards."

"The whole point of us doing this crossover is setting up the fact that maybe next year we can cross over all of them," he said.

Stay tuned. … —

A super crossover

"The Flash" joins "Supergirl" Monday at 7 p.m. on CBS/Ch. 2.

FanX crossover

Dean Cain, who starred as Superman in the 1993-97 series "Lois & Clark" and makes occasional appearances as Kara's adoptive father in "Supergirl," is scheduled to appear at Salt Lake Comic Con's FanX this weekend for autograph signings and photo ops — for $50. Check saltlakecomiccon.com for more information.