Dual batteries

A link to the thing would be nice...........

the marine / fire truck / etc industry has used Off / A / B / Both battery switches for a very long time.

You could do this cheaper and easier, just put the "NHRA approved" disconnect in the common positive line. This of course shuts off everything, but would leave the two connected in parallel.

A second disconnect in ONE of the remaining battery positive would then split the two apart. Those switches are (relatively) cheap.

Lots of ways to "skin this cat."