Well there is no such thing as a 7990 as yet. And normally dual gpu cards aren't as good as a single gpu one. For instance two 7970's would be better than one 7990 (if it exists). Heck, even two 7950's (if they exist haha) would be better than the 7990. And, if you look here:
AMD Radeon HD 7970 review you can see that a single 7970 can run bf3 at 2560x1600 at 33 fps. However that is with 16x Anti Aliasing and the game set to ultra settings. I am sure if you set the anti aliasing to 4x or 8x, the performance would improve a decent amount without sacrificing visuals.
And according to AnandTech here:
AnandTech - AMD Radeon HD 7970 Review: 28nm And Graphics Core Next, Together As One It can run it at 49.7 fps at 2560x1600 on ultra. That is using an lga 2011 setup however. The guru3d setup is on an older x58 setup which explains the large difference in performance. I'd assume normal sandy bridge would sit somewhere in between both benchmarks. So if you weren't stupid and decided to run AA at 8x or 4x instead of 16x I can't see why a single 7970 would suffice. And if it was the overclocked version you can expect almost a 5fps increase on performance as well in certain titles. So you may as well stick with one 7970, at least for now