Yes, stack-able VRAM is almost upon us according to AMD’s own Robert Hallock. Hallock tweeted out earlier this week that the newest addition to the Mantle API would allow VRAM stacking between cards in multi-card setups.
For those of you who don’t know, currently when you take say two 3GB cards and run them in Crossfire/SLI you don’t have 6GBs of usable VRAM, you just have 3. The reason why this is, is because when you are running a multi-card setup, each card is running its own copy of the game individually. The newest rendition of the Mantle API removes the need for each card to run their own copy of the game and therefore their resources can be shared.
This type of functionality will be seen in the future with other APIs like DirectX 12 and OpenGl(glNext) but for now Mantle will be the first to allow it. Although AMD is allowing Khronos access to their Mantle code so it is possible that glNext will be rolling out this functionality along side Mantle.
Now I feel like I should clarify that just because this functionality is available, that does not mean that suddenly all of the games you are playing will be using stacked VRAM. These APIs allow developers more to-the-metal access off the GPUs so stacked memory will only be available in games that the developer optimizes to do so.