600GPH into your sump should be fine. More circualtion in the tank itself should be added however. Most shoot for 15 to 20x turn over in the tank useing power heads or spray bars .
Considering your head you may want to buy a higher flow pump for the sump. The rateings for the pumps you have are most likely without considering head or any elbows if your return is hard plumbed with PVC
. With four feet of head you may be able to use both of those pumps together and the head will reduce the flow from both down to where the OFB can handle it but IMO
that set up is risky and may require installing a ball valve in the return to back off the flow if its to much for your OFB
If you look on the box when buying a pump there should be a graph telling your the flow rate at various heads. In your case you want one rated for 600 GPH
at four feet of head.
I would stick with one pump at that rateing due to the reasons I mentioned in my previous reply.