Let's say you're on your White 2 version. You go to Giant Chasm and get Kyurem-W. Now you have to trade Zekrom over. We encounter the same problem as before.
Now if you trade Zekrom over BEFORE you go to Giant Chasm, then you will have both Zekrom and Reshiram on the same game. You go to Giant Chasm and get Kyurem-W. But if you reset to get the other Kyurem without saving then you can't still have Kyurem-W.
The only way you can do it, is by having Zekrom, Reshiram, and 2 Kyurems on one game. It would be the only way to get both Kyurem forms.