Completely reflashed Ubuntu Touch ontop of working CM 10.1 install WITHOUT WIPE -> No issues
Reflashed CM 10.1 and booted into it -> No issues, no files or settings gone
Reflashed Ubuntu Touch (quantal-preinstalled-armel+maguro.zip) -> No issues, no files or settings gone
So it is not a "real" dual-boot but it gets the job done.
That will continue to work as long as Canonical doesn't require us to completely wipe our devices.
Oh, and if we want to make Ubuntu Touch run flawlessly we should start contributing to the upstream project
instead of doing our own custom ROMs and not contributing back. That way everybody benefits.