FishFrik
Aquarium Advice FINatic
As far as two powerheads vs one...IMO flow is flow and the skimmer does create a current in the tank, so it could count as part of the gph, just like you would count the flow from a sump as part of your flow equation.
I wouldn't be worried so much about that. I had 4 pumps: 2 maxijets 600, 1 maxijet 400 and a pump that came with my CSS "125" in my 29G. Didn't have any heat issue. If you are so concerned about heat, don't reduce flow just to lower the heat, get a couple of hang-on fans.