Mix two video tracks with iMovie on OS X
This type of alignment/syncing is why the clapboard was invented! Without one, you'll have to be a little more meticulous when you manually align the clips.

This post on the Apple Support forums suggests a few helpful features to use:


The basic feature you want to use is cutaways.
Audio waveforms can be used for visual alignment.
Playing audio from both tracks simultaneously lets you hear when the tracks are in sync.
Changing the time scale and showing the frame count allows you to make fine-scale adjustments to the clip positions. You can use the arrow keys to nudge the clips.


Read the aforementioned post for some more tips.