If you have the system as shown in Post #10, the two batteries serve 2 loads: 1. the starter (and engine which includes the alternator) and, 2. the boats electronics. They can be combined (in parallel) for emergency purposes.
As I understand the description of the operation of the Bluesea rig you posted, it appears the two batteries are charged simultaneously which, would, of course, puts the same voltage level on both batteries during the charging process. So, yes, separate VMs would be redundant.
However, the ACR does, apparently, isolate the 2 from each other when OFF or if one or the other needs no charge. So, in that case, 2 VMs would be indicating separate voltage levels (i.e., Charging level or normal battery level values).
However, given that I have no idea how the system might, eventually, be wired (there are other options), I'd suggest the following:
One VM and a 3 position switch wired as below:
Just check the batteries as you see fit, always returning the switch to "OFF".