I've had this happen once before, but we have two heads, two holding tanks, and two macerators so no embarrassing need to visit the neighbours. Having a single black water tank certainly changes the redundancy factor.That can seem like a very long night or two when the holding tank is full.
Most of the Caribbean where is sails have no limits on head discharges whic may be counter to our sensibilities. Macerators are infamous for breaking. The last thing you want to do is have to use someone else's head on another boat (which direct discharges) because yours doesn't and your tank is full because of a broken mascerator.
As the saying goes -ask me how I know.
I had no idea that the 40.5's two heads share a common holding tank. That's got to be a long run to the tank for one of the heads. I can see it would make sense to make the one furthest from the tank direct discharge through a Y valve, otherwise I'd consider an electrosan since both heads use the same tank.