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

Alternatively I know I could use a step up, or 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. So then alternatively 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 12v (or 24v) 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