Okay, so in pokemon Black you get Reshiram.
And in pokemon White you get Zekrom.
If you buy pokemon Black 2 will you get the Zekrom-Kyurem mix
or the Reshiram-Kyurem mix?
Pokemon Black 2: Black Kyurem (Zekrom-Kyurem mix)
Pokemon White 2: White Kyurem (Reshiram-Kyurem mix)
If you look at the box art it shows Black 2 having Zekrom/Kyurem instead of Reshiram in the previous Black and White 2 having Reshiram/Kyurem instead of the Zekrom in the previous white.
So basically you will get the opposite pokemon than last time. The reason is so that instead of getting black and then white 2 (which sounds weird) you will get black and black 2 or white and white 2 because the games are sequels and you would not want the same pokemon twice.
Well, for now what I know you get the black kyurem in the black 2, so that's what I know so far, hope this helped! :D