On a 430/460 there are two thru hulls in the bathroom cabinet. One is a macerator discharge - anyone know what the other one is? There's a shutoff attached to a hose that runs aft. Its not the ac or a generator pick up so not sure what else it might be?

Thats where the head draws water.

I'm pretty sure the head pulls water from the freshwater tank?

The head draws water in from outside boat. Only the shower and sink use the freshwater stored onboard. As Bobby said,

1 valve for discharge the other for suction to the head.

Thanks.