You need to buy an overflow box. Having two pumps may sound like it'll work, but you will eventually have a flood. The amount of water that a pump moves does not stay constant, little particles and debris get stuck in the pump, etc. Not to mention, the return pump is working against gravity, which reduces flow. Unless your two pumps are perfectly synchronized at all times, you will either have a flood in the display tank or a flood in the sump. It will be impossible to perfectly synchronize the two pumps.
You need to buy an overflow box. It is super easy to plumb a sump with an overflow box -- I did it in my old 55 gallon tank. An overflow box pulls water over the side of your tank and down into the sump. It can only pull water as fast as the return pump returns it, so you don't need to worry about synchronizing the flow of the two devices.