There is a styleguide about Hybrid SACD, DualDisc, DVDplus formats.
What about Audio-DVDs with more then one mixes (e.g. 5.1 mix and hi-res stereo as in http://musicbrainz.org/release/c024d126-200f-49b0-8e3d-3212d1151057).
I would split up the mixes to separate "medium". Is that O.K.?
I would definitely expect all the mixes in the same disc / layer / whatever to be in the same medium. The layer issue is because each layer is a different type of format, not just because they have different mixes.