What you need to worry about is the return pump.
If the return pump is moving more water than can fit through the pipe then your going to dry the sump.
Remeber, in an overflow the amount of water that exits the tank is conditional on the GPH of the pump putting the water back into the tank. You can run both holes, but it may not make things more efficient.
Id choose to run both, and possibly increase the sump, or pump GPH.
(but dont put a big pump on a small sump or your going to have problems)