Trying to run 2 PSU for Mining Rig

[dog2bert]

I have a 640Watt Corsair PSU and was getting low hash rates with my 7970 (around 300mh/s)

I have two other 7950's plugged into PCI-e x16 slots.

The 7970 is on a x1 to x16.
This worked with on PSU, but the 7970 got low hash rates.
So I tried using another PSU to just power the 7970, but now it doesn't show up. The second PSU is jumped to run all the time