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You can match MTB and Road, not just XTR.
Edit: Or you can/could but only with a specific version of E-tube and only if you don't mix and match road front derailleur and mtb rear, but the article is 3 years old, so it might not be possible anymore at all.
https://www.cxmagazine.com/mechanical-monday-xtr-mountain-bike-di2-road-drivetrain-shifters
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and only if you don't mix and match road front derailleur and mtb rear
That's exactly what's going on with that Panasonic. But as explained in the comments, it doesn't work.
@zmjones_ raised this point in the GBP thread: isn't it true that you can't mix MTB and road Di2 derailleurs? How does this work then?
Edit: Found this in the comments: