Thks for your reply.
Second of all, on a planted tank you want less water movement, so you don't mess with the plants co2 absorption, otherwise you could just put the 2 canisters running separately on the same tank (but 4 pipes inside a tank wouldn't loo too great on a ADA style tank).
Are you sure about this? I was always under the impression that you want as much water movement as possible to make sure there are no dead spots and that all gases and nutrients are even exchanged throughout the tank? Obviously you don't want enough that it uproots every single plant in the tank.