PSU 1100w+850w

[ubuntucg]

Hello,I have 3 r9 280x toxic sapphire and 1 msi 7990 ,i need help how to use 2 psu that i could run without issues,my problem:
1)if i run 3 r9 280x on 1100w I get problem system shut downs kill-a-wat shows 1050-1075w i think not enough power
2)if i run 2 r9 280x on 1100w psu and 1 on 850w its ok kill-a-watt says that im use 400-425w for 1 card from 850w psu
3)so 7990 it use much more power than r9 280x i didnt check but i think so,my second psu 850w it turns off system when it reach about 800w usage so i cant use this psu to feed 7990 and on 1st psu there usage is about 820w ,so i have 200w fron 1st and 350w from 2nd psu free how can i manage them how i could make my all cards runing?Is there a way that if 1 psu  doesnt have enough power it will draw from another?Any ideas?

[-droid-]

undervolt, undervolt, undervolt

[NameTaken]

Kill A Watt measures the AC power draw from the wall which is 15% more assuming your PSU is 85% efficient then the DC your PSU is providing.