depends on how you like your tanks.... you could run one big external pump and manifold to both returns... advantage less heat quiet but if leaks its outside... one big internal pump divided easy to setup not as much plumbing but heat into water... or like my setup I run two pumps per overflow (sicce 2) or your case 2x 3 or something in the 750-800 gph range lil hear but if a pump fails I still have a pump running... skimmer get the biggest one that fits in sump look at reviews ask questions here see if anyone has used one.. also design your sump to run a fuge... anything I missed I'm sure someone else will chime in