If I were to crossfire two Fury Xs paried with a H100i/i7 3770k at all stock clocks. I honestly don't OC and don't plan on doing that any time soon. (yes I know my username its old lmao)
Here is Guru3D's power supply recommendation:
AMD R9 Fury X - On your average system the card requires you to have a 600 Watt power supply unit.
AMD R9 Fury X Crossfire - On your average system the cards require you to have a 850~950 Watt power supply unit as minimum.
If you are going to overclock the GPU or processor, then we do recommend you purchase something with some more stamina.