The drivers would be easy to write with directx 12. Its the Rendering engine that needs to be able to identify all of the cards, submit work to them, keep them synchronised and then get the cards to submit the work back to the compositing card.
The whole issue goes back to some of the complaints people were having about crossfire on mantle. Most devs not developing systems that scaled beyond dual GPU setups. An 4 - 8 way Fury x system using SFR SuperTiling would be a low latency and FPS monster
. Its just a matter of the devs figuring out how to code their engine to handle multiple cards in a system.