According to an Apple tech (Menlo Park Mall Apple Store) as well as a 1-800-my-appl(I think that is the number) customer service rep it is indeed possible but I can't personally attest to it on the mac side of things but it is 100% do-able in windows which to me further confirms that it can be done in OSX.
In that case, SLI / Crossfire is confirmed for you in OS X as well. Please test it out and let your fellow forum members know. Thanks.