Here is a copy of the final version of my SQ Encoder...
To use to synthesise SQ from stereo, only the front inputs are used. With the pot turned up a left input is encoded as a left back and right as right back, while center front stays upfront. , 270° enhancement. The other extreme produces no enhancement 0° .
I have found this discussion particularly interesting, thanks.
So to reduce your circuit to the absolute minimum, all you need to create a 270° Synthesiser from two SQ decoders is a single inverter-
I must try this with two Surround Masters!