Apparently yes, DirectX 12 will allow memory stacking. Two or three GPUs will act as "one big GPU" if you can imagine something like that.
IMHO it won't be exactly the same as having an 8Gb or 12Gb single card, however it will be a huge gain in performances, if they're telling us the truth.
More important they are optimized by Nvidia directly and they're focused on getting the better from SLI configurations. This is all in theory because all these gains depends on the single game optimization by the producers to get all the better from DX12...