The Titan black, if it was ever made would essentially be a 6GB 780Ti with double-precision compute enabled.
In all honestly I'd just expect it to be two 770s or at most 780s.
if the 790 is true (the specs indicate BS) I'd imagine the supposed 290x2 (or whatever it'd be called) would be announced as well.
That'd be a card worth considering instead of central heating ;D.
There are rumored specs on Guru3d but like all rumours, you need to take them with a pinch of salt.
Basically it seems like the new "titan" is a rebranded 780Ti with 6gb and improved compute.
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.