Well, I've used the same scene I shot to put two different voiceovers on it, that's the beauty of the program in that you can do stuff like that. You can see an example of that in 'The Hunt for Red Hot Coders' too. The bits where I'm doing the stupid Sean Connery accent on the Russian sub commander are the same scene with different voice overs lip-synched in the post production bit of the Movies, I just exported a movie file from post production and then exited without saving changes, that way I could do the same thing again with different voice overs, all it required was a bit of alteration of the scripting dialogue and timing of the phrases to get them to match the body movements, and it doesn't have to be that great.
You'll notice that I also cheated a bit on that Red Hot Coders movie, with some of the dialogue going over external shots of the submarines, which meant I could do those bits really easily, as long as you don't overuse tricks like that it will be fine.
If you can find a copy of this book, I'd really recommend it, it's got some great tips on composing camera shots and narrative flow:
http://www.amazon.co.uk/Directing-Ca.../dp/0950758221
Chock