You can play it in any order and it wouldn't really matter.
You should just finish up White before you start Black 2, for a couple of reasons.
1. White forces you to use Unova Pokemon only during the story (assuming you don't trade), so it's good to help you get accustomed to the new Pokemon. In BW2 it's expanded a lot so you';ll see a bunch of Pokemon from previous gens.
2. Iirc you need to finish White to gain access to a few extra things like N's Pokemon and the added cutscenes.
3. You're already passed the 5th gym might as well finish it.
After you're done you can just trade everything to your Black 2, but you can't trade till after the 1st gym. Also, if you have a lot of Pokemon in White it will be tedious to trade everything over. Check what you can also catch yourself in black 2 and save yourself the trouble of trading everything over.
3DS FC-2879-0906-2636 name:RaRa