In White 2 I have 2 kyurems and 1 Reshiram and 1 Zekrom.I fused the first Kyurem with Reshiram and got The White Kyurem form, but why can't I fuse the second Kyurem with Zekrom ?
Because you only have one DNA Splicer. It is being used splicing the DNA of the Reshiram and the Kyurem. Source