Provided your switch is a "make before break" design there is no worry moving between 1, 2 or ALL/BOTH just don't pass through OFF.
To test your switch to see if it is make before break;
1- Turn on DC cabin
2- Make sure motor
3- Rotate switch between 1,2 & ALL (try both fast and slow movements)
4- If you see ANY flickering or the lights momentarily dim or drop out buy a new battery switch as yours is not make before break.
Most all switches from companies like Guest, Blue Sea, Perko, BEP and others have been make before break for many years.