Regardless of the internal coupling of the two sets of pots via the one marked S5 the whole array still only seems to connect to the SQ Encoder via two four-way signal busses (one for the Forward and one for the Backward encoding module). This certainly ties up with the following description of the input arrangements of the 4211-I believe that to be incorrect. Look at pan pot S5, it connects to both the forward and backward encoders, that's the only way it will work.
i.e. Two sets of four inputs for the F & B encoders and one set of four inputs for the 'normal/ standard' encoder (a total of twelve inputs).