With all honesty, the chance of the 7990 beating the GTX690 is pretty slim
The 7970 draws 195W, the GTX690 with its 2 GPU's draws only 270W
7970 - http://www.guru3d.com/article/amd-radeon-hd-7970-review/8
GTX690 - http://www.guru3d.com/article/geforce-gtx-690-review/8
The reason why AMD will struggle to beat the 690 with the 7990 is because of power consumption. Remember, the higher the TDP, the hotter the card will run. Kepler is amazingly power efficient which is what allowed the GTX690 to be such a beast. Whilst the 7970 isn't power hungry either, it still consumes ~22W more than the GTX680. The GTX690 is a miracle card because not only does it essentially perform almost identically to 2 x GTX680, it actually uses around 50-60W LESS power. I don't know what kind of black magic Nvidia were using to allow that to happen, but whatever they did, it was an outstanding result.
So, back to the point. Assuming the 7990 is 2 x 7970's, the TDP will be a minimum of 300W but very likely to be 325W-350W unless AMD trim the GPU's, which would in turn, result in a performance loss. The best part about the GTX690 is that it is essentially two complete GTX680's on the one PCB. If AMD need to cut back the 7970's to get a sub 300W TDP, then that will result in performance loss, which means Nvidia wins this time around.
That is why, to be honest, I don't think AMD will release a 7990. More than likely they'll just wait and release an 8990. Unless they design the 7990, accept its not as fast as the GTX690 but price it far more aggressively (say, $750USD MSRP). Then they will have a winner. But if they want to re-take the performance crown, I highly doubt it. Tahiti XT just isn't efficient enough to compete with Kepler in a dual GPU situation.