You add a second audio track, and use a 'set up' menu to select the appropriate track to play back.
For example, you have one video track, but two audio tracks. Button 1 on your menu plays the video with audio 1, and button two plays the video with audio 2. You can do this through using button 'conditions' in the property inspector for each button. It's quite easy. You can also do this with scripts, but it is not at all necessary for what you describe.
You might have different footage for the directors commentary, and want this in a second video track. That's fine too - as long as there is space on your DVD. Simply add that as a second track with its own audio and point to it from a menu.
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lafcpug mod