i would think that the 2 filters would work best if run independent. maybe have each one to run on opposite sides of the tank.
+1 on this advice.Run them separately. But cross the inflow and outflow:
The inflow of filter1 sucks water from the left. The outflow of filter 1 discharges the water in the right side of the tank.
The inflow of filter2 sucks water from the right. The outflow of filter 1 discharges the water in the left side of the tank.
+1I think it has to do with the water circulation/movement in the tank. Water from one side is circulated to the other side.