I would suggest installing a battery combiner rather then the switch. Run
the 24 to start and the 27 as the house. Anytime the engine is running the
24 will charge first and then the 27.

If you really want a switch install a zap stop.

I had a switch and would forget to switch etc-etc just like everyone else
with a switch.

By the way if you switch off with the engine running the alternator diodes
will blow. That why the zap stop.