Problems mining 2 r9 280x

[Roman1974]

Im newbie, i started mining with one r9 280x, and all ok about 750 speed, but when i install another r9 280x problems comes.
I use cgminer 3.7.2 and catalyst driver 13.10, when i have the two graphics, one works ok 750 but the other only run 60 aprox

I tried cgminer 3.7.0 , 3.5, i update catalyst driver to 13.12, but always the same. im using windows 8.1

I dont use crossfire phisically and not active on software.

i tried to change from pci-e, i tried many many things but no works.

Any idea to help? thanks

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[Roman1974]

Working ok,thanks