It's a linked stereo compressor, so the gain reduction is the same for the L and R channels.
If you follow the schematic the signal from the L and R is summed near the attack and release controls
A dual mono version would allow the L and R to be unlinked.
To make your SSL dual mono you could add a second set of attack / release controls and then a switch so as to not sum the CV for the Gain Reduction.