Hello Robert,
if you right-click the merged movie you should see an option in the
context menu called "Split parts into independent movies". It's not an
undo in the sense of what you might know from e.g. ms word, but it will
unsplit the merged media files into individual movies.
This feature is already present in Movito v2.
Feel welcome if you've further questions or ideas for improvements.
Best,
Holger
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Movito - The better movie manager
http://www.movito.info