Offizieller Kommentar
The digital output is affected by the channel volume levels, not the master output - so in effect, you could run the digital output from mixer 1 to mixer 2 via digital and still have independent volume control for each set of analog master outputs on each mixer.
Realistically, 99.9% of people wouldn't notice that it's doing a D>A>D>A conversion vs. a single D>A conversion.