Nice lay out.
Normally, when your Ignition switch is turned on, it sends 12 volts to your meters. This is most likely how your setup now then you might prefer.... to not be switching other buttons etc to get your v-meter on or off and/or view things and they're off when the key is off, all on when key is on.
This is similar to how mine is set up(see attached), standard setup, no messing or mounting additional types of switches etc or cutting into or un-doing your existing start batt. meter wiring.
If you accidentally ever drain your Start batt. enough such that the relay doesn't energize (needs about 8 volts min) so as to view/monitor your ACC batt. voltage, I suspect you'll be having to use it anyway with your "add a batt" switch to get started(***or try to get started) and switching a good batt. in circuit will turn it on obviously, so it's a moot point.
***Adding a severally discharged(or perhaps shorted) batt. in parallel with a good/fully charged batt. with these "add a batt" switches is not the best practice BTW......nother story, nother time..
Clean, simple setup, and you can view both gauges simultaneously, hands free at just a glance while running or whatever....Just a personal preference, i don't like to have to be switching things around, or as few as possible when perhaps it's rough and things get crazy as they certainly can and do.
Off with key off, on with key on. $5 relay (any 12VDC SPST, or SPDT w contacts rated at say 100/200 ma or higher is fine too, I just picked a 3A DPDT, spare contacts are always nice to have) and an additional gauge but hey, sometimes convince costs a few bucks, looks like you have Faria? so around $30 new......This relay is socketed for quick change out but, will probably outlast the boat
Looks like you have plenty of room and pre-cutouts to add a matching gauge too or go with a dual.......Options, options
View attachment 219316
and now for the Tweedle brothers show........