The overflows will work the same with one pump vs 2. The return pump flow does not have to match the drain flow within a single overflow. As long as the total flow from the return pump is less than the combined overflows can handle, then you will be fine. Think of it like this:
1) You have NO return flow, just the overflows that drain to the sump.
2) You dump a bunch of water from a bucket into the tank.
3) The water will "over flow" and go down the drain to the sump. It doesn't matter which drain works faster or slower, the water still flows to the sump. It doesn't matter if you dump the water closer to one drain or the other. It doesn't matter if you dump the water slowly or fast, as long as it is not too fast for the drains to handle.
A pump from the sump back to the tank works the same way. It doesn't matter where the water comes from. The pumped water raises the water level in the display tank, and this water spills over into the overflows.
Hope that helps.
Jack