There was a diagram at Jensen-Transformers of a "Passive Stereo Matrix for M-S Mic Technique" that might work:
http://www.jensentransformers.com/as/as065.pdf.
I was decided to test it, but then I bought a Wendt X3wich (has which, although it is not a Mid-Side decoderbut) , has 3 mic inputs, so I use an adapter cable to connect Side (figure of eight) mic to input inputs 2 and 3 with phase inverted phase, if necessary.
