They just require power. Some folks disconnect the signal wire and run the stern planes without the ability to manually override with the transmitter. Mostly useful when the sub has both bow/sail and stern planes.
Norbert Bruggen uses two levellers to control roll and pitch on the Dolphin and Deepflight Aviator models. They are mounted at diagonal angles. If you take a look at his website, and download the PDF instructions for the R/C Dolphin, it shows the layout there.