Although when planning the holes you take into account all the above factors, when looking for a return pump, you are limited to how fast the tank can drain. If the pipes are 1" O/D and 3/4" I/D then each will drain at about 600 gph
max. If the pipes are 3/4" O/D and about 1/2" I/D then your combined flow will be under about 800 gph
if memory serves me on that one.
The first is likely the answer as its a common drain hole size. So plan for about 1000 gph
max return to avoid pumping out your sump faster than the drains fill it. Then you have to take into account head loss which is the difference between the water levels in the tank and return section of the sump. Pumps normally have a graph on the back of the box which shows rate vs head loss. Add 1' for each 90 degree elbow in the return between the pump and tank.
Whew. Good luck.