Both Mantle and DX12 can combine video memory

Robert Hallock (Head of Global Technical Marketing at AMD)  shared something interesting on Twitter earlier on. You guys know that when your have a GPU graphics card combo with dual-GPUs that the memory is split up per GPU right ? 

Thus an 8GB graphics card is really 2x4GB. As it stands right now, this will be different when DirectX 12 comes into play and apparently already is with Mantle. Basically he states that two GPUs finally will be acting as ‘one big’ GPU. Here's his complete quote:

Mantle is the first graphics API to transcend this behavior and allow that much-needed explicit control. For example, you could do split-frame rendering with each GPU ad its respective framebuffer handling 1/2 of the screen. In this way, the GPUs have extremely minimal information, allowing both GPUs to effectively behave as a single large/faster GPU with a correspondingly large pool of memory.

Ultimately the point is that gamers believe that two 4GB cards can’t possibly give you the 8GB of useful memory. That may have been true for the last 25 years of PC gaming, but thats not true with Mantle and its not true with the low overhead APIs that follow in Mantle’s footsteps. – @Thracks (Robert Hallock, AMD)

There is a catch though, this is not done automatically, the new APIs allow memory stacking but game developers will need to specifically optimize games as such. An interesting statement.

Keep in mind though, this is a marketing rep talking ...