I had a similar issue when trying to use two HIS Radeon 7850s in an MSI 990FX-A GD-65v2 motherboard. What happens with me is that only the Primary card is recognized...period...in Windows or Ubuntu. My XFX/HIS 6850s work no problem, as do my GTX460s, but for some bizarre reason my MSI board won't recognize more than one HIS 7850. Looked online for answers and found that MSI boards tend to have issues with non-MSI cards. From what I found, it's very rare, but much more likely than any other board manufacture.
In your situation, you can try updating the BIOS. I have the same board on one of my Intel machines, and run two Radeon 7930s on it very reliably AFTER I updated to the latest bios.