Almost everything about multi-gpu is theoretical as there are literally only a few games that implement support. Tests have been done showing yes you can use cards from red and green teams. Again though how well this would work out in wide practice is anyone's guess. For instance with Crossfire or SLI the games that support it were few overall and it never really had what anyone could call wide adoption. Less than 3% of PC gamers ever used more than one card at a time. The amount of AMD gamers would then be less than even a half percent of those gamers. Now that the developer of the games are responsible for building support into their DX12 engines it just doesn't seem to be happening. Why? Well there really isn't enough of a market to justify the extra trouble. PC gamers are already a very small percentage of who they code for and multi gpu users barely register as a statistic. Most gamers today look for a single card solution that will meet their needs. It really is the best way to go to have stable, more trouble free experience. Of course you will have those that say 2 cards worked for them just fine, and that is true some had good experience but that is not opinion of most that went down that road.
Now if you do decide to try mult-gpu they say you should still use cards of similar performance levels as likely the will perform more to the lower common denominator but I have no idea much testing has really been done on this. You can find some video with testing of "Ashes of the Singularity" that test different cards. I have seen absolutely zero information of testing with Navi based cards. I don't think Vega II supports it either. I know it doesn't do Crossfire.
I would love it if someday a standard comes around that doesn't require the extra work at the driver or developer level that would allow us to add our cards of unequal pairings and get better life out of these expensive purchases. Here is to hoping that happens, but don't hold your breath.
