Question Adding an External combiner box

Does an external combiner box combine amperage or voltage?

I wish to add another string of panels to my system (4 strings already) to bring the total to 5 strings.

I have two 18kpv's

Currently forty 370w Aptos bifacial panels separated into 4 strings of 10 panels. I also have 164,000 watt hours of mixed manufacturer batteries all LIFEPO4. 

String 1 into main (input 1) MPPT of inverter one.

String 2 into second (Input 3) MPPT of inverter one.

String 3 into main (input  1) MPPT of inverter two.

String 4 into second (input 3) MPPT in of inverter  two.

I originally had both strings 1 and 2 going into the main input (slot 1 and 2) of inverter one and strings 3 and 4 into main input of inverter two.

I separated them so I could monitor individual voltages of the individual strings.

I know the main MPPT's of each 18kpv can handle the input since it combines them together internally. 

My wiring is oversized (8 g) so not an issue. 600v underground burial cable.

Im adding twenty 100w panels set for 55⁰ for winter and separated by 40 feet inorder to help in avoiding my wintertime shadowing issue. These panels should produce 424v, still within specs of the second MPPT in input 3 of inverter one.

I wish to use the combiner box on strings 1 and 2.  Doing it this way would save me running a new set of wires 150 feet as well as upgrading my conduit to a larger size to accommodate my new wiring for the additional string.