I'm not sure what you mean but I'm guessing a combined filtration system where one pump filters many tanks.
I've posted this somewhere on this forum but forgot where. Anyway here's one way you could achieve a series of tanks.
A: The tanks; you could use any number or sized tanks as long as the top level is equal.
B: Is the water pump which doesn't have to be very big since bettas do not need big currents. If the total length is around 1.5 meters, a 1500L/hour pump is more than adequate for bettas.
C: Are the filters; you can use a double filter system by placing an internal filter at the intake and either an internal or an external filter at the other end.
D: Are pvc tubes/pipes made into a U shape - the pipe size depends on the size of the tanks. 1/4" pipes are adequate for tanks up to 3g. But bigger is safer because it wont create big currents. They function as siphons to move the water and wastes from the far end to the pump. Be sure to cover the bottom with mesh or something so bettas don't get into them.
E: PVC pipes to carry water from the pump to the far end/filter. Make holes above each tank so they will have water surface movement which helps dilute more oxygen in the water (IMO). The number/size of holes needed depends on the size of tanks and pump. The bigger the pump/water flow, the bigger/more holes you need to make.
You can also make a parallel system, but that would be more difficult since you have to make holes in your tank/tub wall and fix tubing in such a way that they don't leak - IMO this will give you a hard time when you need to clean them. Further you need to make gravity equal to the pumps power to avoid over flowing - or make the holes rather big so it would flow any amount of water down.