My friend, I was showing you that Adobe Premiere is shown to work on 2 identical cards. I am not sure whether it works on 2 different cards. But it works on 2 of the same and it scales very well, so that is promising. The article also points out that crossfire doesnt need to be enabled for OpenCL compute which further bodes well.
I'd say that bodes well for your 7790 addition. Give it a shot and report back if it helps, it wont hurt your system to try and put it in. I'd be interested to hear how it turns out