I have 2 R290-X cards and they are working fine, I run them in single card mode most of the time as I have 2 monitors connected one monitor to each card and that all works great.
If I then decide to use crossfire for gaming my machine enables crossfire and that takes me down to just the one monitor I have selected to run crossfire on and it lets me know I am in crossfire mode but I would expect to get a framerate increase in my games when playing them with the vsync off. I get absolutely no difference in framerate at all, it is basically the same as I got using just the one card.
I am running an AMD system built around an FX9370 Chip, 32 GB ram, 500 gb SSD as my C drive, a 1250 watt PSU so power should not be the issue.
Can someone please help me sort this problem out as I have a £400 graphics card in my computer that is useless when using crossfire.