90MileBeac said..why not set the thrusters parallel with the center fin to eliminate drag.
Because drag is reduced when the fins are in the direction of the water flow. And the water flow is not parallel to the center line... and the angle varies with speed. Thus, side fins being positioned for surfing speed, they will add drag for flatwater paddling. On the opposite, parallel fins will add drag when surfing.
Ideally, you would want specialized side fin boxes with a toe-in designed for flat water paddling, or boxes with adjustable toe-in. But it i simpler to just go single for flat water.