Marine toilets aren't supposed to hold water
A manual marine toilet brings water in on the upstroke and pushes it out on the downstroke...there's no way to bring water in and keep it in the bowl unless you pour it in with a cup. The most that should be left is about an inch in the neck of the bowl, which will seep out. So will any water poured into the bowl with a cup...it's only a matter of how quickly it does it. When the joker valve is in good condition, it seeps out. Any faster, and it's time to replace the joker valve. A joker valve is a one-way valve...it isn't supposed to prevent water from leaving the bowl, it's only supposed to prevent backflow from flooding and overflowing the bowl. A brand new joker valve will sometimes close tight enough to prevent water in the bowl from seeping out, but that only lasts for few flushes.A marine toilet CAN be jury-rigged to hold water, by removing the joker valve and installing a high loop in the discharge line so that flush water between the bowl and the top of the loop runs back into the bowl. It's fairly common on houseboats (those floating mobile homes on barge hulls that litter the inland lakes and rivers), but I can't imagine doing that on a sailboat 'cuz all the water in the bowl would slosh out all over the head when the boat heels or in heavy seas. That's WHY marine toilets aren't designed to hold water...they're designed to be used on boats, not in condos.As for why the aft head gurgles when the forward head is pumped, without seeing how your sanitation systems are plumbed, I can only guess. Two toilets that far apart SHOULD be two completely separate systems--as separate and individually complete as they'd be on two separate boats. There are maximum distances that any toilet can pull in flush water or push waste, and the distance between fore and aft toilets is WAAAY too long for both toilets to share ANY plumbing, holding tank, or treatment device without causing all kinds of problems. But it sounds like a previous owner may have run them both to the same holding tank...or worse yet, connected 'em both the same intake thru-hull. I strongly suggest you do two things: get the owners manuals for your toilets from the mfr and read 'em. If they haven't been rebuilt in the last couple of years, it should be done (and while you're in there, lubricate both pumps with a good slathering of SuperLube). Otoh, depending upon what make/model they are, it may be just about as cheap to replace 'em as rebuild 'em. The article in the HM forum reference library (on the forum homepage) will be useful to you too, when it comes to maintaining both your sanitation system(s) and your fresh water system.