AMD's Robert Hallock revealed that Mantle and other low-level APIs such as Microsoft's DirectX 12 finally make it possible to perform memory stacking on SLI and CrossFireX configurations. The new low-level APIs will allow two GPUs to finally act as one big GPU, which will result for example in two 4GB cards giving you 8GB of useful memory.
The only catch though is that this is not done automatically, the new APIs allow memory stacking but to achieve this game developers will need to specifically optimize games as such.
One of the more interesting features of multiple GPU configuration is how the system itself works. The reason why there is no memory stacking effect is because a complete copy of the game has to be run in each GPU, and frames are alternated between whatever number of cards you have. However, with low level APIs like Mantle having now entered the scene, the developer is granted the coveted ‘to-the-metal’ access to the GPU. That means, that with the proper optimization developers can now surpass the limitation of the old soft architecture and enable buffer stacking.