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Yes, you can use the polyphonic expansion on a CS6X.

From the PLG150-AN manual:

POLYPHONY : 5 notes maximum (latest note priority; polyphony is expandable *1)
*1 Depending on the particular "mother" device, up to eight additional
boards can be installed, for a total of 40-note polyphony. On the CS6x,
for example, two boards can be installed for a maximum of 10 notes; on
the MU128, three boards can be installed for a maximum of 15 notes.