Does this mean the differences are only in the canonicalization of unicode values?  IDNA is a wire protocol, which means that an application can't know if it is being asked to decode an idna1 or idna2 string unless there's something in the protocol that tells it.  But if the differences are only on the encoding side, and an idna1 decoder will "do the right thing" with the idna2 string, then that would be interoperable.  I'll have to read the standard, but I don't have time right now :)

idna is a codec:

>>> b'xn--mller-kva.com'.decode('idna')
'müller.com'

(that's python3, it'll be a unicode string in python2, obviously).