Yes you need two DS's. Activate the memory link on your Black/White 2, and use DS Download play(like you would do for migrating from 4th gen to 5th gen.) with the DS with your Black/White in it. Thats my understanding.
Sorry about the leaving you blank on how to do it This site here gives you full detail how to do it. You have to access memory link from the Unova link in the menu(Black/White 2)