I just did this for a video of my niece's senior voice recital. (Video is below)
I had three audio possibilities. A CD recording, the side camera audio track, and the back camera audio track. I decided to use the side camera audio track with heavy use of muted cutaways from the back camera. I decided the CD recording had too much noise even after fixing in Audacity. And the back camera audio track was OK but had too much coughing and noise.
Here are tips for getting it lined up perfectly.
1) In iMovie/Preferences make sure that you check "Display Time as hh:mm:ss:frames" so you can edit to the frame level. (Also, select "Show Advanced Tools so you get the Cutaway feature)
2) In the iMovie Project View, set the thumbnail slider to 1/2 second so you have a great deal of control.
3) Initially, leave the volume at 100% on the Cutaway track, so that both tracks are playing. You can hear when they are off.
4) Click the "Waveforms" button in the Project view, so you get a visual cue of how to line up the tracks.
5) Between the audible cues and the visual cues, you can nudge the cutaway back and forth one frame at a time by dragging until you have it perfect.
6) When it is lined up to your satisfaction, set the volume slider of the Cutaway track to zero, and turn off the waveforms. You can also move the thumbnail slider back to 5 seconds or so.
7) For some of the fine work, I switched to the "single row timeline view". This slows you down a bit, but lets you see the waveforms more clearly.
8) I made heavy use of the iMovie Comment Marker feature. I watched the whole base video/audio track and inserted comment markers at the points where I wanted the Cutaway to begin and end. For example, a comment might say "Insert Cutaway 1". I would usually time the cutaway so it started at a distinct audio event, such as when the piano start. This comment marker made it easy to drag my cutaway to a point that was approximately correct.
9) If you need to do a lot of cutaways, I find that it helps to use the SPLIT CLIP command in your project. This way you can have a single cutaway per clip, and this makes it easier to manage and is less processor and memory intensive for iMovie. I would use the comments to mark the entry and exit points of the cutaway as described above. I would then select that portion of the clip that included these markers plus a little more, so that a yellow border is around the selection. Then right-click/SPLIT CLIP (or CLIP/SPLIT CLIP in the top menu). This does not physically splt the clip within the Event. It only virtually splits the clip in the project.
