If there are 2 separate stand alone units, such as a primary house with a basement/carriage house/detached garage with it's own kitchen and bathroom, then I would simply list them as 2 separate units on the OTAs. I would be transparent about this and post the link for the other extra unit in the listing for each and sell it up as a benefit for larger groups. Big groups will be more than willing to book both. Otherwise, be sure that each unit is private enough in regards to noise control, access, parking, private outdoor space, etc so that separate groups are not forced to co-mingle if they don't want to. You would probably not want to have a huge spread in guest numbers between them, because a 8-12+ guest place is probably going to make too much noise for a mellow couple in the ADU, unless the units are far enough away to essentially be different properties.
Otherwise, I have thought about the exact scenario above for a 2 bedroom with 2 masters suites. Give 1-2 guest parties the option of renting a single room and keeping the other guest area locked via smart locks, then unlock the other bedroom with 3 guests for more and charge more for it. Could be a great idea and would allow you to have a lower cleaning fee to be more competitive. I think that this could be doable with the recent push by AirBnb to upgrade their single room offerings.