it is possible.. for example:
--Raising tank 2 level:
1.5Q of water flowing out of tank:
0.5Q pushes up into tank 2, filling it up, 1.0Q goes down to the pump
--Lowering tank 2 level:
0.5Q from tank 1, 0.5Q from tank 2, combine for 1.0Q to the pump
doesn't have to be "two directional flow"...