Not sure what the second tank is. To prevent the pump from running with any small water usage, it is common to run the outlet of the pump to a pressure tank (a bladder tank). There's a check valve so once filled, the pressure is retained. The pump control switch is place by this tank and typically has a 20# difference...turns on at say 30# and off at 50#. To prevent it from running the supply tank dry, a simple float switch could be used to interrupt the current to the pressure switch.