This is usually because the MIDI out and MIDI in of the Nova is connected to a sequencer which is remapping controller messages (generated by editing parameters on a Part) to a channel corresponding to the channel of the other Part. The solution is to switch the local control parameter in global to off. This means all edits will be carried out via the MIDI chain rather than directly and the sequencer will dictate which part is being edited by the MIDI channel setting on the track. The alternatives are not very friendly, they are to filter out controller messages in the Sequencer, or disconnect the midi out of the Nova, or disable the 'Thru' function on the sequencer.