Some people use a Y-splitter into the sub for that.
Do you mean splitting the TRS-RCA adapter on each output so there will be both a L+R input into each sub?
So Output 3 -> TRS-RCA adapter -> RCA y-splitter -> L and R input on sub (presumably summing internally)? I'm not sure how that would help since I would still have to reduce the gain on the output to match the subs to the main speakers. Unless I use the on-board gain controls for the subwoofers, but I was trying to avoid that since it would probably introduce more noise vs doing it all inside the Flex (also makes level matching and configuration reproduction more tricky). But I suppose if it comes down to it I could try to rough hand level, mark it on the plate, and then do some measurements and do the last bit electronically.
I'd still like an expander though so I could make at least a rough loudness curve