mixing stereo and mono dialogue
 

Is it generally considered faux paus to mix together dialogue recorded in stereo with dialogue recorded in mono? I'm working on a project where the dialogue was recorded both with the on-camera mic and with a shotgun going into an MD. While in general having superior quality, the MD audio is sometimes unusable because the boom operator was either producing noise or misdirecting the shotgun. I want to use the stereo, on-camera audio in those places, but I don't know what problems will arise from doing so.

I suppose it would depend on the kind of project...

A conventional approach would be to convert any stereo dialog to mono. You could do this by mixing L+R, or by discarding L or R. Different NLEs give you different tools for doing this.

See how it sounds. Good? Good!

Usually the on-camera mic would be in the mix for ambience efx, which might benefit from stereo.

You'll also need to work carefully with equalization and levels at each edit point so you there's no jump in level or timbre at the edit to reveal that two different mics were used. Depending on the distances involved you may also need to slide the sound back and forth slightly with respect to picture to correct the slight sync differences that will occur because of the fact that it took longer for the sound to travel to the on-camera microphone than it did to reach the boom mic. 30 feet of distance is one frame of video sync so the sound picked up by the camera mic has a slightly different relationship to the action from sound picked up by a boom close in to the scene.