I doubt The 790 will be 2x 770, the 690 is 2x 680 and it would basically be around the same as the 690. Saying that though, Nvidia do like releasing cards that are only slightly faster but for more money, so you never know
2x 780 or 780Ti sounds more likely, especially if AMD were to release a 290/x x2 Nvidia would lose out if they only did 770 x2.