EffClef,
The front and rear output jacks are wired in parallel and this wiring is not effected by the mono / stereo switch. If you are using the mono output jack in the back to drive a mono amp (pre or integreted), then you can use the front Mono input for another non Series bass.
You'll probably have to adjust the volume of the amp or the bass to compensate for any differences between the output levels of the Series and the other bass.
Cheers,
Peter Jonas