If I may suggest, stick with one return pump, but one good one. The reason being that if one pump fails, the other pump will not be able to keep up with the flow of returned water and you have yourself a disaster = flood.
I have a 125 gallons Jetstream dual return/overflow tank from Marineland and I use a Mag24 pump in-sump. Yes, it does generate heat but helps to warm up the water so the heaters don't have to work as hard.
From the information I have read, for this tank, maximum overflow rate is about 600 GPH
for each overflow, giving you a theoretical limit of 1200 GPH
wich is very adequate.
Personally, I would suggest that you add a few powerhead pumps inside the tank for the additional water movement.
That's my opinion. HTH