I would recommend not joining the overflow pipes into a single, blockable tube. Unless the tube receiving the two outflows is larger than the delivery tubing you will be choking your return. Additionally, combining the tubing allows for a single point of failure if blocked by debris or an energetic (voice of experience) snail.
If you have decided on a single return pump, check into using a
SCWD (or something equivalent) to still give you return flow from two different directions.