Currently I have a Bitcoin computer working fine, mining for a few eeks.
1x 5870
1x 5850
I just received another 5850 and PCI-E risers. I connected the GPU and started the computer. How ever windows hangs at the booting-logo. I had latest ATI drivers installed and computer was fine before. Removing the second 5850 lets windows start fine.
Any ideas on whats the problem?
Have you tried booting with both cards separately? That should give you an idea of whether one is bad or not.
If they both boot individually I'd suspect power supply as the poster above said.