well whilst it does work, theres no point risking compaitibility when you can usually get 2 cards the same (unless you're buying used). If you are buying used then definatly dont, you don't know what settings have been changed, is there a new BIOS on one of the cards or something. It's genreally kinda silly. And If you can afford 2 970's, you should be able to afford a 980Ti, which is better, and you should always buy the most powerful single card with your money, because later on you can get a second one, whereas you'd have to buy a new gfx card all together otherwise.
Basically it's not really worth it, unless you already have one 970, which you dont because you have AMD.
Just get a 980Ti, or if you really want 2 x 970 then get the same dang cards and save yourself the hassel.