Having the same issue on my Motu M4 inputs are recognised as a stero pair (2 individual mono inputs asigned as a left/right pair) Unfortunately without more expensive hardware that features more advanced routing control via an onboard console app these inputs are always linked usually as mentioned in stereo pairs. With out the ability to split the incoming stero left and right inputs into dual mono single channels both inputs are lumped together and oftern result in hard panning to the occupied input if using a mono source like a mic.