Can you use SLI on a CrossFire board or can you use CrossFire on a SLI board ?
Well , in general , the answer is NO. But it's said that if you hack the drivers , you can use SLI on a CrossFire board or CrossFire on a SLI board.
Caution:There is no guarantee that if you hack the drivers then you use SLI on a CrossFire Motherboard or CrossFire on a SLI board, so do it at your own risk!
If you can return 1 piece GTX280, let's do it. Otherwise Either you need to buy a mobo with 780i / 790i for SLI or just hack the driver as well as Maxiar quoted but it would be with high risk that you need to re-install your Windows if any serious conflicct occurred.