Same here, three way or "Y" valve as Stu says, either but not both... The aft tank is higher than the port side so if you open 'em both, the aft will drain into the port side .. Lots of folks use only the port side tank unless going out for a long time. A dry aft tank makes for a faster boat.. On trips where that aft tank is needed, most folks use it first then switch to port tank ..
some folks have made a little manifold instead of the three way valve.. A manifold would allow either or both to feed the pump. Aft is supposed to be 30 gallons (250 pounds) and port is 35 gallons (290 pounds)..
AS my three way valve failed, it began to let air from the empty aft tank get into the pump suction causing a lot of spitting at the faucets.