Because of the sounds that I use, mostly prey sounds of distress and Fox vocalizations, I use only the front speaker. The rear speaker is designed for sounds of larger critters, like if I were to call elk or deer or mountain lions, bears with bear cub sounds.
Plus it is easier to play the sound cone(similiar to playing the wind) if it is only coming from one speaker. An incoming predator will align himself in the sweet spot of the sound cone, it's strongest-loudest-clearest.
Uses less battery power if only one speaker is on. Hope this helps!