As long as they should not render together this is not a problem. But it looks like that VIA want to build a software solution to make this possible. Actually that is not difficult to do. If I had the hardware it should be easy to build a working prototyp in a week or less. The main problem will be games with fullscreen postprocessing if you want to use a SFR technic and games that use anti latency methods will make trouble with AFR. But nVidias SLI have to fight the same problems.
Megabyte's(who was bought by alienware) sli method was also able to work with different graphics cards. It was designed to use two pci cards or 1 pci and 1 agp and the performance of the game would be limited by the slower one.