I would not expect the switch/pump to pump wiring to be ample enough to support 2 impeller pumps. On the 6 pump setup, id suspect that each pump's harness is home run from the pump to the switch/relay output.

So adding supplementary pumps to an existing single pump system, you would want to supply each new pump with its own harness from the source.

As too the switches. Whether these are traditional mechanical DPDT rocker switches or touch screen controlled through the helm system, I would not want to parallel a 2nd impeller pump to each output. This can be 60A peak and about a 40A constant current draw. This will overload a Carling rocker for sure. I would convert the switch to a SPDT and use relays for both pumps.

If touch screen controlled, there is a good chance they are using a relay assembly to control the polarity reversal of the pump and the relay packs low side input is controlled by the PSB helm system. This would be most ideal for adding that 2nd pump. Just need to wire the other relay assembly input in parallel to the existing pump relay assembly input.