Question about 2nd PSU in one pc.

Hi all, I really need some help here, just got a new motherboard and 4x R9 290x graphic cards. My real worries are the 2 differents PSU'.

XFX 1250W and the 2nd one is a Be quiet 650W. I was wondering if i could use the XFX to support all of the graphic cards and let the other PSU handle the the Motherboard and fans etc.

i hope someone can help me out with this. Got a ATX entender but i dont really know where to cut and what to do to get this to work. Dont wanna destroy my system.

Thank you