Draw power from one of two batteries in series

However charging may differ for the individual batteries.
If the 24V supplies a preamp, and the 12V supplies a power amp, the discharge will be significantly different between the two batteries. If recharged in series, the current required by the more-discharged battery will have to pass through the less-discharged battery. That is probably a situation to be avoided, as the charge current will continue well past the full-charge point of the one battery.
I would definitely do some research on the possible effects. Maybe a switching circuit or bipolar charge circuit could provide a workable solution.
 
If the 24V supplies a preamp, and the 12V supplies a power amp, the discharge will be significantly different between the two batteries. If recharged in series, the current required by the more-discharged battery will have to pass through the less-discharged battery. That is probably a situation to be avoided, as the charge current will continue well past the full-charge point of the one battery.
I would definitely do some research on the possible effects. Maybe a switching circuit or bipolar charge circuit could provide a workable solution.

After reading about a bit, as you said it gets a bit more complicated, as it seems this tiny imbalance that will happen between the batteries will not be corrected when charging them in series, and so the difference between the batteries will only grow larger and eventually become a real problem over time and could result in overcharging...