One of the biggest limitations of adopting a multi-GPU configuration such as AMD's CrossFire X and NVIDIA's SLI is that, if two or more graphic chips are paired together on the same motherboard, their video memory would not get added up since both cards would be trading the work load across the system. In simple terms, two or more GPUs run as a separate copies of the primary GPU and frames of the application alternate between each graphic card. 

However, with AMD's Mantle and Microsoft's upcoming DirectX 12 API (application programming interface), gamers will no longer be running into this issue as the video memory on multi-GPU configurations will be added up to allow gamers to immerse in titles that utilize additional amounts of video RAM.

According to the tweets by Robert Hallock, the Head of Global Technical Marketing at AMD, two graphic cards working together would not have been able to stack up video memory in the past couple of decades but with the latest API, that barrier might become a thing of the past. 

The extract from his Twitter feed states:

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 and its respective frame buffer 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 that's not true with Mantle and it's not true with the low overhead APIs that follow in Mantle's footsteps".

While the option to stack up video memory will be possible for gamers who own an AMD CrossFire X or NVDIA SLI configuration, it will be non-existent until game developers start including the feature in their gaming titles.