I have White and White 2, if I trade my Kyurem and Zekrom from White version, can I fuse them into Kyurem-B, put that in my box, and make a Kyurem-W with the Kyurem and Reshiram from White 2?
Actually you can.
First, get whatever your version's mascot is. Black 2 would be Kyurem-B, and Kyurem-W for White 2. Then, PokeSav the version's opposite mascot into you boxes. Then you're done. So you can only do this with cheats.
No, because it is only your in-game Kyurem that works w/DNA Splicers.