I looked on the schematic for the SZG-ADC-LTC226X.
I know the SYZYGY spec says pins must be populated in order - not leaving any unused pins between used pins. It seems that as per the schematic above, that also applies to unused differential pins, needing to be populated with single ended I/Os, before the single ended I/Os are used.
Are there no cross talk issues that creep up when doing this, and is that the required setup (ie - unused differential pins must be used as single ended and filled before the single ended I/Os are used)