I think the main reason to go with a dual motor scooter is if you want something more powerful or need to really strong hill climbing. The biggest drain on your battery is going to be how fast your ride, how quickly you accelerate and # of stops/starts. Taking it easy in dual motor mode on a dual motor scooter shouldn’t be too much less efficient than riding the same way on a single motor scooter.
Regarding operating a dual motor scooter in single motor mode for to conserve battery:
The unpowered motor, of course, is going to add mechanical resistance just from the friction which should be pretty small.
However, spinning the unpowered motor is also going to cause it to act like a generator and it will send a current out toward the controller. Depending on how the wiring is setup in single motor mode, you might get a back electromotive force that creates additional resistance and this could be pretty significant.
We haven’t tested to see how dual motor scooters behave in single motor mode with respect to resistance on the unpowered motor, but make be worth it to look into it. We’ve gotten some questions like this before.