Although this is a fairly old thread, I thought I'd respond anyway. My R33 has what you speak of - Sea water comes in the thru hull and has "T" then bronze valve before going on to cool the engine. The other part of the "T" has a bronze valve before going on to the sink. This system allows me to have one thru hull for both and I can shut off each independently. When the engine is off I close it's valve and if I think about it - when motoring for long distances - I will shut off the sea water to the sink - just an extra safety thing. You could shut off the sink valve every time you start the engine if you wanted.
For my two bilge pump - one switched - one auto - I use two thru hulls and two hoses.