Fallout 4 is not a good optimized game
For 2k gaming 60+fps you would need 980ti , and you won't even be 60+ stable
Fallout 4 doesn't support any multi-gpu configuration , only 1 gpu is supported
crossfire 280x can get you great ultra 60+ fps on 2k , but not on unoptimized games like fallout 4
having crossfire for that game is useless , you will play on 1 gpu only ....
Also 280x doesn't support freesync , the monitor will work just fine but you won't be able to use freesync feature on 280x