On my Four Winds 28Z there are also two valves on the black water tank. One at the discharge nozzle (along with the grey water tank) on the driver side, and another on the opposite side of the coach, just below the black water tank itself.
While there is no mention of it in the manual, I am speculating on several reasons I believe it is there.
First, it may be so that the tank can be more efficient. When you dump contents into the black water tank, especially on initial use after flushing, there has to be a certain amount of water in the tank for the bacteria to efficiently break down the contents. If there were no valve in the passenger side, it would take several additional gallons to fill the black water tank enough to function. Many people may simply not otherwise initially put enough water into the tank to recharge it after dumping.
As well, there are two types of bacteria that can be found in a tank - anaerobic and aerobic. Aerobic bacteria - is the good bacteria, and does not smell (a relative term), but only forms in the presence of oxygen. Anaerobic bacteria is the bad bacteria, and is the stinky stuff, and forms when aerobic bacteria does not - in other words, when there is no oxygen present.
So I am sure the second purpose of the valve on the passenger side is to keep anaerobic bacteria from forming in the 8ft or so of sewer line that goes across the bottom of the coach as there may not be sufficient access to oxygen should solids tumble down the sewer line.
Third, if there were a lot of solids in the black water tank, they might "pack" themselves into the sewer line if they were allowed to congregate at the distant end. Keeping the passenger side valve closed would prevent this.
A fourth reason I suppose is if the "crossover" sewer line is kept empty, there is less weight loading on the line, which may lessen stress as you are rumbling down the highway. Afterall, if you look at the support hangers on that line, it is not exactly supported all that well.
I would not be one bit surprised if Thor started putting the passenger side valve on these coaches after the sewer lines started falling off. I have no info on this one way or the other, so it is just conjecture on my part.
What I do is close both valves before use. Then when I dump, I open both valves, in no particular order. Also, if I run out of tank space a day before we leave the RV park (and before I dump), opening the passenger side valve allows a couple more gallons of capacity.
The only thing that works on a RV is the owner...