I am really looking forward to this episode. I think it is clever idea to have these two meet up.
And, the episode is named after the comic of the same name where heroes would crossover. World's Finest. I am also glad they finally came out and stated Supergirl and Flash are on different Earths (Earth 3?) and they even had an appearance of Supergirl in a flash-sideways when Barry traveled to Earth 2 in an earlier episode. As I stated before, Supergirl can't exist in Flash/Arrow's world due to Superman playing a big part in her history.
By the way, Livewire and Banshee are the team up villains for this episode.