They have built in controllers. That is one of the reasons they cost what they do. Also if you get the wireless versions they talk to each other and work in concert. One can be a master and other slave which is cool for wave generation and for things like going to feed mode and night mode.
I would probably put one on each end, even with the overflow on one side. The randomness and chaotic flow you get with 1 of these things on each side of the tank is awesome.