question

You have made it unnecessarily complicated.

Wire the batteries as one bank and set the amp hours correctly. Don't have a 2nd house bank or ESSENTIALLY you have a separare system and it is unecessarily -unnecessarily- complex, which is ALWAYS bad in a boat.

Add a starter and use the aux lead on your shunt to get voltage.

You CAN wire all grounds together and ALL loads of course and drop the ground right in and ground the whole ocean just like a counterpoise on a SSB -the BMV only measures the completed circuit through whatever the positive lead finishes the loop -like your radio or something. There is no circuit in the ocean to measure so it won't do anything per say but you could start picking up on voltag leaks and need to recalibrate far more frequently than.on land -unnecessarily complicated!

Just think of things as loops with the BMV only measuring whatever positive load is on the other side of that shunt as it finds "ground" and common ground being whatever you connect BACK to your negative bus bars and battery (ALL loads).

It will never work correctly for SOC with a switch because the two separate banks will never exactly parallel each other except in a theoretical universe.

It gets even worse with lithium or anything mixed at all but exactly the same batteries all around and they should be wired as simple as possible for function.

Get a second monitor and decide on a simpler system that won't be so complex and potentially break easier. Or wire it all as one house.bank -you don't want to be using starter batteries for house loads and house batteries to start except in emergencies. unless everything is lifepo4 and then why have them separate at all?

Simplify

Or get an second completely correctly set up system and monitor. I can certainly understand the desire for ultimate flexibility via a switch but then why use an expensive BMV vs simple old-school volt and amp meters? You use the BMV because it offfers those special functions like a SOC! But your setup negates these features! So maybe you would be better served with simple volt meters...? A few days on the water and you would know how to read your charge states pretty quickly.

You could get a lot of heavy marine wires and gauges for the cost of that BMV!

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If money is the issue and you really can NOT combine the banks and can't get a 2nd BMV there is a similar monitor mademin China, by Hangchen? It is NOT a rip off of victron gear and not as cool as the BMV and not as good for flooded batteries but it would work. It's harder to get calibrated and lacks the coolest victron functions but it would work for someone on a tight budget. You would need to keep the systems separated as usual. Complicated.

@Stefanesai

The BMV 702 offers comprehensive monitoring of a single battery bank and basic monitoring (i.e.) voltage of a second battery.

Since the shunt 'counts' power in and out of a battery it is not possible to have accurate monitoring or two banks even if you did manage to safely make a way to switch between the two.

As @Trina mentioned it is better to have two monitors or settle for the basic voltage monitoring option.

https://www.victronenergy.com/battery-monitors/smart-battery-shunt.

This is also an option if you don't need the face or extra wiring.