So many different solutions out there.
Something like this is a good example - https://www.powerbox-systems.com/pro...mpetition.html
Plug 2 batteries in and the powerbox treats them as individual batteries. If one battery dies the other takes over.
Lots of other brands like AR, Jeti, Futaba, Emcotec etc.

Here is another variant from AR -https://boomarc.com/en/ar-reg-switch
This one is a simple battery backer. It just depends on what setup you are going for I guess.

Oh and I know everyone has probably told you this but get rid of the NiMH.. LiFe or Lipo are heaps better.

PaulH.