Well it looks like you are in Luck, I just spoke with someone from EVGA's support team and they said that even though it is not listed/supported, you can xfire BUT there is no guarantee that it will perform to the same standards as SLI...
I don't understand why EVGA would do such a thing when it is better for the company to document this info for sake of the users wanting to buy an EVGA board and not deal with Nvidia....
Would be nice to see if there is at least one person that can confirm the performance of xfire on such a board....