That will be pretty difficult to set up using single contact C/Bs.
If the batteries are in parallel, whatever fault condition trips one CB will trip both anyway.
Can you mount the C/B's so you can see the trip indicator?

You could try connecting only 1 meter with the meter +ve on one battery +ve and the meter -ve on the other battery +ve
If both CB's are closed the meter should read 0.0x.
If a C/B opens the voltage will drift from zeroish as the battery terminal voltages change.
-ve voltage if one CB opens, +ve if it is the other