I have four thru hulls on my transom.
1) The primary/maintenance bilge pump. It is a gray water diaphragm pump which gets me down to 1/8" of water in the bilge. The diaphragm pump is a series of check valves so it does not need an external one.
2) A 250 gph centrifugal pump for those Oh S**T moments. It is wired separately from the smaller pump
3) My manual Whale gusher pump for a backup or added capacity
4) My water HX output for my cold plate
The biggest issue I have is fitting three pumps/strum boxes in my bilge. It's pretty crowded in there, but I am very happy with the arrangement.