Overflows are a method of taking water out of a tank and to a sump.

It is important to note that it is not possible to set up a system of two pumps, where water is pumped from one tank to another using a one pump, then from the second back to the first tank using another pump. It is impossible to balance the flowrates of the pumps so that they exactly match. So in all cases will at some point in time end up with water on the floor.