I made a graphic explanation although it doesn't look as good as the one in the OP as I used LibreOffice Draw. As the picture says I've not tested it but if you look at a DPDT on-on switch datasheet you can check this yourself: Switch position 1 connects the pins 1 & 2 plus 4 & 5. As pin 4 is connected to the plus pole of the first battery an pin 5 to the minus pole of the second battery and there is nothing connected to pin 1 this is the only connection and equals a serial connection of the batteries which means you get the 18 V. Switch position 2 connects the pins 2 & 3 plus 5 & 6. Pin 2 is connected to plus on battery no. 2 and pin 3 to plus on battery no. 1. Pin 5 is connected to minus on battery no. 2 and pin 6 to minus on battery no. 1. So this is the parallel aka 9 V mode to safe battery life. Check with a DMM before connecting to the pickups. Don't have one? Get one! A cheap one will be sufficient.