I also have two tanks. If connected similarly, both tank lines attach to a valve tee (furtherest back). Turning it determines which tank is operating. There is another tee valve before the pump and it only goes up and is vented. I always leave this valve closed to the vent or you will just suck air. I think it might be used if the backflow valve in the pump fails when on city water. If that happened, you would fill your tank/s when trying to use the city supply and have to disconnect.
It also may be used for winterizing but we don't do that here.
I have a FW main tank and a galley tank. I also have two gray water tanks. The galley for the kitchen, main tank for the shower and bathroom.
Bob and Better Half
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