I think the overflows are rated at 600gph each for a total of 1200gph. This only means that you can max out your flow into the sump up to this number. Therefore, you need to buy a pump rated around the same to pump water back into the main tank. Your overflows in your main tank will always match your return pumps output as long as its under the overflows rated flowrate. in other words, your return pump dictates the sump flow rate.
The skimmers rating is how much it can handle and effectively do it's job. It doesn't relate to the sump flow rate. If it's placed in the sump, it's input is from the sump and it's output is sent back to the sump for no net gain or loss.
When you are trying to establish a "total tank turnover rate," you can use the flow through the sump and any powerheads in the main tank. If the rest of your mechanical filtration is in the sump, then it doesn't get added into the equation.
Hope that helps.
Ryan