I was a little worried that the pump out suction might damage the flapper valves in my overboard pump, but so far, so good. Instead of a Y valve, I have a simple T that joins the waste tank(flexible tank) outlet; the deck pump out fitting; the overboard waste pump.

During pump out, my overboard pumps jockers close(without the damage I wondered may happen), and the dockside pump out pump drains the tank. So far, so good.