Who said he bought the cards 2 days ago, the way OP is making it seem is that he already had that rig and had 1 7990 and just bought a second and was waiting for it, if he has 1 card then it makes more sense to crossfire them than just buy all new cards. In my personal experience using the 7990, if you have antiquate air flow in your case, even in crossfire, the 7990 does not run too terribly hot.
haha thanks for the helping trying to explain what my situation is atm, i had a 7990 for a while now, i bought another one because i had some extra cash. when i game i normally game at 1440p just because it seems to run smoother. So with an effective 3gb frame buffer it handles it just fine. Plus i only hit about 65c when im under full load (mind you they are only running at 50% when playing BF4 on ultra, minus AA) so the second card will just add to the performance and make it easier on the first card.