Post

STMix can function as two mono mixers if you dummy the R inputs of the left inputs you want to mix separately from the rights.
It will create two distinct outputs to L and R outs.
L1 X
_ R2
_ R3
L4 X
L1 and L4 go to Lout, R2 and R3 to Rout. You can add the Aux unattenuated to either.
It’s a bit of a workaround but my point is you get a lot of bang for your buck with this module in a very small space (6hp).