If you knew the only thing wrong with the card were the ports than yeah you could xfire and display via the other card. But you wouldn't know what's actually wrong with the card without spending money on another 280x to see. Unless you know someone who has one to test, then sure. It's your money after all. 280x's in xfire would require 750w psu as well, 500w between the cards and then some for cpu motherboard drives etc, so maybe more expenditure there for another psu if current isn't adequate. Then there's the lack of xfire support in games too to think about.
Maybe something more current would be better depending on the rest of your specs. Im sure you'd find something relatively cheap that would beat 280x in xfire.