Sims 4 crashes a lot when Crossfire is enabled. Desabling it resolves pretty much all crashes to desktop for me. Not that the game needs the dual card horsepower, but still shouldn't crash.
I don't know, maybe a lot of advances have been made in support since you wrote this but mine works fine with 2 r9 390X's. Again it does not need the extra power but it is fun playing the game at 90-100fps!!!
Edit: forgot to add in ccc turn off "frame pacing" most crossfire related crashes and fps issues will be eradicated.