Although I am using GUIminer on Windows, I am using 2 7950's so i can give some insight here. I have tried mining with both crossfire on and crossfire off (just a flip of a switch with the control center). I have noticed MAYBE a 5Mh/s difference between the two, which i consider irrelevant. I also use this computer for gaming, so Xfire on is really the better option for me anyway. The cards are both HIS 7950 IceQ2's OC'd to 1150Mhz. I get just under 600Mh/s on each when i'm not running anything else. Your GPU1 will always run hotter, just because of the thermal dynamics of having 2 cards stacked so closely to each other. If you are using MSI afterburner (which I highly recommend), set the user fan progression so that you can keep temps of GPU1 sub 80c -my made up heat threshold. My curve is set so that if card is 70c, fan is 75%- 80c, 85% etc etc. Remember, through Afterburner I've Oc'd both cards from 950 to 1150Mhz so your results could vary noticeably if you opt not to go that route.