It can be done with an analog matrix, i.e. op-amps, potentiometer, maybe a mixer if you can set it up. If the SS is adjusted with an equal mix of mid and side, (presume a 12 o'clock setting of the Width control), then matrix is:
L' = 0.5 * (L+R) - m*R
R' = 0.5 * (L+R) - m*L
where m is the remote mid-side mix ratio, and 0 <= m <=1.
The output of this matrix L' and R' become the L and R inputs to the SS.