Both heads drain into the top of the tank (on the side/aft corners) and the top of the tank has openings for a level sensor, pickup tube, and vent (foil covered 1" hose). The pickup tube is cut at an angle near the bottom so there's always a couple of inches of effluent in there, unfortunately. The pickup tube goes to a T which connects to the pumpout deck fitting on one leg and the overboard discharge pump on the other leg.

Tom, a reason we don't need to worry about damage to the discharge pump valves is that we'd never use the deck pump out while the seacock is open. There's a limit to how much vacuum can be created in the discharge hose considering the tank is vented.

BTW I'm using a Snake River gage, and the sensor uses vacuum pressure in a tube to measure tank level. That's good because it doesn't clog, and it works fine at-rest, but the system is slow to react to changes and doesn't give a reliable reading while the contents are sloshing underway.