Brian Gaff has brought this to us :
If it were a low power use I'd say isolate them by diodes and charge one at a
time but in this case, either get a battery capable of starting the vehicle
on its own, make sure but batteries are switchable and keep both charged and
use them in rotation.

You cannot isolate batteries via diodes, because it would create a
difference of 0.7v due to the voltage lost across the diode. 0.7v is a
lot of difference in battery voltage terms, much more than the
difference between a fully charged and a flat battery.