question

You essentially have 2 x 12V battery banks connected in parallel (with each bank consisting of 2 x 6v batteries)

So I assume that you already have a link cable between the +12v positive & the negative between the 2 banks.

Accordingly the voltage & current readings reflect BOTH banks / all batteries together.

In reality there would be a slight variance between each bank (if you were to disconnect the link cables & measure each bank separately), but this delta would be small unless a significant imbalance exists.

For the 'mid point' measurement to work you will also need to link the mid points of the 2 battery banks - this point should be at +6v.

Ensure that you use a sufficiently large sized cable to make the mid-point link (to protect for a worst case imbalance condition/failure when significant current could flow across this cable), unless you add a fuse in between.

Then you can connect the B2 lead from the shunt to either of the center battery terminals.

See extract from the Victron quick connect guide (which is the same setup as yours except this example has 3 banks in parallel instead of 2);

Another option (which is what I do) is to have a 4 position battery switch (off, 1+2, 1 only, 2 only) and connect the output to B2, & inputs to the mid point of each battery bank. The normal operating position for the switch is 1+2 - so the switch acts as the link cable.

With this switch you can momentarily 'disconnect' the mid-point link & INDEPENDENTLY check the mid-point reading for each bank.

This is a very handy feature & not hard to achieve. I then fuse both connections to the battery switch separately at the battery.