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One thing Crossfire works in Window mode or borderless when Running Mantle.. So maybe when DX12 comes it will also work.
Its also worth asking what would be the reason to why you would want to do this anyway??
Some games don't run in true fullscreen (Heroes and Generals) I like to run in borderless window so that I can have web browsers open with guides and things on 2nd and 3rd monitors while playing (Elite Dangerous, trading tools etc).
On my old AMD rig none of that would have been possible, with SLI it works fine.
It swings both ways tbh
Why not use Web browser in steam overlay? Or run Full screen game on main display and have the browser on the other screens with tools etc
AMD is much more flexible on what GPU you can mix, where Nvidia its not.. Can you match a 970 with a 980?? can I match a 290 with a 290x?
etc
SLI vs CF which is better supported.
Totally impossible to answer and I don't think there is a person who could know either.
The reason I say this is there are thousands of games available and loads of different PC configurations to run them on. Unless it was known how every game on every possible PC configuration ran it is impossible to answer.
OP the question you should have asked is how does games X, Y and Z (depending on which ones you are interested in) run in CF or SLI.