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I'm doing the same. But listen
Soon you will have to transfer them to X/Y. So there is no point in having a living pokédex in BW2 if you already have it on B/W unless you want to finish the pokédex.
So I just caught one Purrloin and using the infrared thing and 2 DS, it's really easy to transfer one by one, you dont need more pokémon. Transfer Purrloin for Bulbasaur, the Bulbasaur for Ivysaur and so on