Cheers! Er, that looks a little confusing. And - SPOILERS!!Brian, you've probably found your answer by now, but if you want, here's a decent breakdown of the different crossovers, including Supergirl and Legends of Tomorrow. There are a couple of crossovers, with the main thing being the set up for the Legends show.
List of crossover episodes
That's great - I'll try to ensure I have both 2nd discs for the new seasons at the same time, then.Okay, let me see...
Episode 2.8 Flash and 4.8 for Arrow is a part 1/part 2 crossover (Flash first, then Arrow) that sets up Legends of Tomorrow. That's the only important one. There are a couple of appearances of Arrow characters in The Flash and vice versa, but I think that's the only key one. (Sorry about the spoilers; wasn't thinking!).