If you want to swap between your acoustic and electric, you may need an AB-Y box.  Plugging something into the 'Power Amp In' will keep the amp's sound from coming through.   You'd have only your acoustic sound until you unplugged the cable.

So, run the 'Pre-Amp Out' into A on the AB-Y box.  Then, run the acoustic into B.  Run a cable from the AB-Y output to the 'Power Amp In'.  Then, you can switch between the two.