Since using the ring terminal of the jack to switch the batteries would result in a finite resistance across the battery that sits 9V above the ground potential, since the "tap" between batteries will not be switched; it would not be a good idea to use a standard wiring scheme.
As I see it, there are two options.
1. You can run the bass at 18V and use an LM7809 or LM317T regulator chip to create a 9V or 9.6V (Fresh battery voltage.) power supply from the 18V. You may also use one of the smaller 100mA regulator chips in the TO-92 package, if TO-220 is too large in the cavity for whatever reason. The disadvantage to this may be the excess current draw required for the regulator chip, but it should still be low current. Thermal dissipation is not significant, to require heatsinking, so don't worry about that.
2. You can switch both batteries with a DPDT mini toggle. The disadvantage of this is the need for a dedicated power switch, and the loud popping that will occur when switching.