Bilge pump choices
I also have a rule 1500 gph pump with separate float switch and dedicated through hull. This is located in the main bilge but not the deepest part.
I have installed a "smart" Rule 500 gph pump in the deepest part of the bilge. It has a built in float switch and will turn on when the water level rises slightly. Where the hose is attached, I put about 1" of 3/4" tubing to bridge the output lip on the pump and a 3/4" to 1/2" L-shaped connector. After making this connection, you have a 1/2" fixture to which you can vertically connect 1/2" tubing. I then run the 1/2" tubing to a separate (existing) sump drain fixture elsewhere. I dont actually mount the pump, but lower it down as far as possible into the deep bilge using the vertical 1/2" tubing. It has worked very well.
You don''t want to T into the existing bilge line. If you are underway and the pumps are running - you will only be pumping out water at the same rate as the single pump at the electrical expense of two pumps. You also don''t want to add another through hull if you don''t have to. Is it possible to lead the new pump output somewhere else?