The 690 runs the two cards separately, so yes, there are 4GB of ram attached to one slot, but no, that doesn’t mean you get 4GB of potential contiguous footprint, each of the two GPUs only has access to 2GB.
It’s literally two 680 in SLI on a single slot, nothing more. Except they are also ever so slightly under clocked.
So, as far as we’re concerned in DCC, the 690 is worse than a 680, and effectively gives you only 2GB of ram for normal use, and in any GPU rendering supporting SLI, all your textures will have to be loaded twice, with consequential synchronicity costs, race condition prevention, bus busyness etc.
Putting two of those together, would still only really give you 2GB of actual usable footprint. So all you would be doing would be wasting 4 times the money instead of double the waste
These are cards meant for large monitors that take dual link, stereoscopic game rendering, or immersive triple monitor setups, where the redundancy of data would be required anyway.
For anybody else, including gamers running games on a single monitor, it’s a huge waste of money.
A dual 690 would be the apex of pointlessness, on par with a gold plated Ferrari.