I have two devices which need to be battery-powered (remote area, no grid power available). One device requires 24 V, the other requires 12 V. Both are very low amperage (~50 mA).
 
Can I use my two 12 V batteries in both series and parallel to provide 12 V to one device, and 24 V to the other? 

Alternatively, I know I could use a step-up or a step-down transformer, but at least one of the devices monitors and reports battery voltage (maybe both, not sure yet), which I would lose with a stabilized voltage output. If I have to use a step-up/down converter, is there a type of transformer which has linear voltage output based on source input voltage, instead of a stabilized 12 V (or 24 V) output? If so, what is it called? And is it more efficient to step up, or step down in this case? 

Here is a simple drawing showing what I'd like to do:
 
[![enter image description here][1]][1]


  [1]: https://i.sstatic.net/ISGZR.jpg