Wiring Two 12V Batteries for Both 24V Wheelchair Motors and 12V Starter—Is It Safe?

Im making a sawmill with power feed driven by wheelchair motors. The sawmill has a gas engine, so it has a start battery, 12v of course. The wheelchair motors are 24v, but i have been running them in 12v off the engine battery. My question is, can i wire 2 batteries in series to get 24v, to run the wheelchair motors, and run the starter off of one of those batteries directly in 12v? in researching, the only issue i have found is with the batteries not drawing evenly, but im only using the 12v as the starter power, shouldnt it be fine? and what about charging from the alternator? will it charge both if one is wired directly? should i add interupter switches to change from series to parallel? keep in mind im looking for the easiest, cheapest solution. and i can always keep running them in 12v, with reduced torque and speed. 

I think you grasp the problem well.  Yes, you can put them in series to get 24 V but starting from just one at 12 V will unbalance the state of charge between the two batteries.  To address this, I think the best, and actually simplest, solution would, indeed, be to switch the batteries to parallel for starting and series for running.   Be careful though not to mistakenly start with 24 V.   It would be good if you can wire it in a way to make that impossible.  Best.  -JD