This is the limitation behind the traditional alternate frame rendering - AFR - technique used today. Which in turn negates the need of having to mirror the resources in the two separate memory pools of the GPUs. Or even render different stages of the frame and pass the rest onto a different GPU. This is because instead of tasking each GPU with rendering a whole frame on its own, each GPU would actually render different parts of the same frame using Split Frame Rendering, SFR for short. Essentially creating a 16GB, 5160 GCN core behemoth. So if let's say we have two R9 390 cards in the system, under this mode both GPUs can be combined to form a graphics engine with twice the memory and twice as many GCN shader units. It allows for the computing/graphicsc resources and the memory of multiple GPUs to be combined into one larger addressable unit.

Linked mode is only available when there are multiple near-identical GPUs in the system, similar to what Crossfire and SLI setups look like today. The first is dubbed Linked and the second Unlinked. There are two types of Explicit Multi-Adapter enabled through DirectX12. So in theory DX12 games, if programmed so, can take advantage of AMD Radeon R7 integrated graphics processors or Intel HD integrated graphics engines in addition whatever discrete GPU or GPUs are installed in the system. This is possible under DX12 because the API allows developers to address each graphics processor in one given system separately and assign tasks to these GPUs independently of one another. This essentially means that users should be able to run AMD Radeon graphics cards with Nvidia GeForce graphics cards at the same time in a 'Cross-SLI' configuration if you will. This promise was made a very long while back but it looks like that what Microsoft had promised with DIrectX12 is actually coming to fruition with Oxide's upcoming DX12 enabled Ashes Of The Singularity real-time strategy game. One of DirectX 12's many promises is to enable mixed configurations of AMD and Nvidia GPUs to work together through the explicit asynchronous multi-GPU feature.

Hybrid crossfirex configuration