Hello,
When connecting the output in stereo, Voice A will only be output to the Left channel an Voice B will only be output to the Right channel. This hard panning is fixed and cannot be changed. The Pitch Mix will determine how loud either the A or B channel is.
When connecting the output in mono, the both voices will be output to a single channel, and the Pitch Mix will act like a A/B balance parameter.
The Diatonic algorithm is the same on the PitchFactor and the original H9, but the algorithm has improved tracking and key/scale quantization on the H90 and the new H9.
Let me know if you have any other questions.