And either way, why or why not? I'm curious if Flash fans are trying the show out (maybe for the second time) or if SG fans are interested to see if Gustin will grab them.
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EW article w/ some spoilers:
The Flash-Supergirl crossover airs Monday, March 28 at 8 p.m. ET on CBS.
On the possibility of an expanded crossover:
http://www.ew.com/article/2016/03/18/supergirl-flash-crossover-spoilers
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EW article w/ some spoilers:
The Flash-Supergirl crossover airs Monday, March 28 at 8 p.m. ET on CBS.
So how exactly does the crossover work? For the uninitiated, Barry/The Flash, who has been training to increase his already supersonic speed in order to fight übervillain Zoom, uses a tachyon device that causes him to run so fast he literally ends up in an alternate 
universe. Therein lies the reason why it’s a lot easier to bring Barry Allen to a different Earth than the less plausible un-super superhero Oliver Queen (Stephen Amell). While the trip will amount to a hot 
second on the April 5 episode of Flash, it results in Barry spending a day in National City (and on CBS for the March 28 episode of Supergirl), where he’s shocked to discover another hero. “Supergirl has never heard of The Flash and Flash has never heard of Supergirl, so there’s a lot of like, ‘Wait, you’re the who, now?’ ” says Kreisberg. The comedy, in that sense, hits the ground running with the classic tale of boy saves girl, girl miraculously flies away, didn’t need saving in the first place. “Barry and Kara take a little bit of a leave of their problems for a week to engage in the kind of hilarious high-stakes high jinks that ensue when two superheroes wind up in the same universe,” Kreisberg says.
On the possibility of an expanded crossover:
Hopefully close enough that they’ll need a rematch next season, maybe with Green Arrow and the Legends as an audience. “That’s in the cards at this point,” says Gustin. “The whole point of us doing this crossover is setting up the fact that maybe next year we can cross over all of them.”
http://www.ew.com/article/2016/03/18/supergirl-flash-crossover-spoilers