Sanitation plumbing is the weak link
at all boatbuilders. As far as the sanitation system is concerned, to production boatbuilders everything but the toilet is just added expense that adds no value to the boat...I've yet to see a production boat on which ANY thought has actually gone into designing the sanitation system. If they even understand that a holding tank isn't a water tank, they don't care...cost is the deciding factor. So they buy the tanks from the cheapest sources, put them where they'll fit--never mind that it will have to be replaced--or at least re-hosed--someday, requiring taking the boat apart to do it...they connect two heads at opposite ends of the boat to one tank even if means 20' of head discharge hose from one...use water hose as vent lines, etc. Or, believing that odor is unavoidable, they run 20 vent lines from the bow to the transom to send it aft of the boat--thus guaranteeing odor..etc, etc etc. The short answer to your question is: two vents cost a LOT more than one...not just the thru-hull and the hose, but also the labor time to install a second one.
