You really need a 2nd battery monitor and then configure a 3rd to sum the 2. Is there a RSSI analog input pin on the FC? From the HWDEF it looks like there is on Pin 8 but not really sure with that version you have. And I’m not sure it will work but worth a try as one potential way to do it.
How do you have the batteries connected? There would be another way with a common Power Module.
Then try what I suggested. Connect the 2nd ESC current pin to the FC RSSI input and configure Battery Monitor 2 with current pin 8 and see if you have a reading on battery monitor 2.
Or, go buy a power module.