A bunch of AMD slides published by WCCF Tech reveal the advantages DirectX 12 offers when you have more than one graphics chip in your PC.
With DirectX 12, developers can more easily offload graphics work to a processor with integrated graphics and if you have a second video card game will be able to let each GPU render half of the frame instead of using alternate frame rendering as is done with DirectX 11. The new technique is more efficient and should eliminate stuttering issues.
The slides also reveal the ability to combine memory pools:
Something else new mentioned in these slides is the use of combined memory pools. It used to be that in order to keep both of the GPU’s working together, everything in the workload was kept in the memory of each card, so none of the workload was divided. In DirectX 12, however, the memory pool can be shared across all GPU’s, so that if the frame being rendered by one is particularly memory hungry then it can access the RAM of the other cards if necessary. While we’ve seen GPU RAM usage spike with recent games, now all the assets will be put in the shared pool, accessible by any GPU available, even integrated solutions.