Hi, Im having the same problem./help plz...  My pc wont start up .  Everything works perfectly fine with 1 xfx r7990 but when I plug in the 2nd gpu my whole mobo fails after a few sec, restarts, and begins to run but does not give video output. Again works perfect with just 1 xfx r7990 but cant figure out why it wont accept the other...(or any gpu for that matter.  Have also tried the r9 280x with the same outcome)

my mining rig specs:

cpu- i3 4130
mobo- msi Z87 mpower max
memory- (corsair-CMX8GX3M2A1600C9) DDR3 1600 1.65V 4GBx2
Harddisk: 1x 500 GB SATA
PSU- (thermaltake) Toughpower XT Gold 1475W

with of course...GPU: 2x XFX R7990 6GB
like to connect all 3 utilizing each PCIe slot. (r9 280x as well  but fails just like the 7990)

plz and thank you for all help