Q. Can DR128s or DR66s be linked to increase the number of inputs/outputs yet remain controlled as a single unit?

A. DR128 and DR66 do not feature digital interconnection between units, so there is no access to the digital equivalent of the mix buses.
It is possible to combine mixers in the analogue domain by feeding the same input signals to two DR units in order to give you more outputs, for example, but the units would need to be treated as separate units for the purposes of programming and configuration.