This topic comes up occasionally, and at first it seems like such an obvious solution that everyone begins to wonder why all builders don't set-up their systems like this at the factory. But they don't typically do it this way, probably for a variety of reasons.
One issue is that sink drain seacocks/thru-hulls are usually much larger than the head/engine intake sea-cocks. So there could be a concern about constricting the flow, especially if any debris goes down the drain. If that drain/intake gets clogged by sink debris, it could take the toilet partially out of commission.
Also, drain thru-hulls usually are not as far below the waterline as intake thru-hulls. If your head intake thru-hull is a foot or two below the waterline, you could easily end-up with a lot of "standing water" in your sink drain. This may lead to as much or more stink than you are trying to cure by giving the head a fresh water rinse.
One more concerning is healing. If your head intake/sink drain is closer to the centerline of the boat, and the sink is outboard, the sink can easily end-up below the waterline, or even below the intake/drain when the boat is healed. So if you open the thru-hull to flush the toilet, you had better have a way to prevent the water from gravity draining up, into, and out of the sink.
So think it through a bit before taking the plunge. We've had good results with simply adding water to the head (we have the same shower arrangement as Christyleigh, but a cup of water from the sink works almost as well).
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Pacific Seacraft Crealock 31 #62
NEVER CALLS CRUISINGDAD BACK....CAN"T TAKE THE ACCENT