What version of the AMD driver are you using? Anything before 16.50 will not work.
Part of the problem could also be the client. The GPU slot detection and assignment is not good with different types of cards. I have the same issues in Windows and Linux. For Windows I had to force it to use a custom config.
Can you post the first 60 or so lines of the F@H log, where it lists all the system stats and the config.
Driver is 16.60. I found the problem, it is the same problem I had a few weeks ago on another machine. When FAH starts the WU, the driver segfaults.
I don't have my logs anymore because in frustration I installed windows. YOU KNOW I had to be frustrated to install windows. j/k I like windows, linux and bsd all the same, like children. Except one of my children is no freaking good. (I think I need meds)