Today CBS released this great mock up comic book/poster for the upcoming cross over of The Flash on Supergirl. The only thing I know if from this short synopsis. I figured they were going to have to be from an alternate Earth since that's been the theme in season two of The Flash and that way they can explain why she just can't show up in The Flash or Arrow universe at anytime. I'm really looking forward to this cross over as I really enjoy both of these series. And from the set pictures that were released it looked like the two main actors had a blast.
Here's the official synopsis: Kara gains a new ally when the lightning-fast superhero The Flash suddenly appears from an alternate universe and helps Kara
battle Siobhan, aka Silver Banshee, and Livewire in exchange for her help in finding a way to return him home.