Don't think it's supported.
from a review of the game itself:
If you're sporting multiple graphics cards, you're unfortunately out of luck—Origins won't use currently more than a single GPU. Considering the CPU limits imposed by the engine, that's probably for the best, and this is one more major release in a growing list where SLI and CrossFire won't do you any good. That may change in the future, but I wouldn't count on it.
Support needs to be added by ubisoft, last I heard was not yet.