You would need to do this in a video editing program such as iMovie (though its separate handling of audio and video may be limited) or Final Cut Pro. Try asking in the iMove forum. It's not something you can do with Automator by itself.
Sorry, I should have been more specific. I mean ones that were split for transmission with a split/join program. In windows I know I can join them from the command line with just a simple file append.
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Combine audio/video files
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