Thanks for your reply. No was the answer I was expecting, but figured I'd ask
incase anyone had come up with a workaround.

I realize updates can be pretty fast, but that's not always the case,
especially if you are dealing with a site that has a lot of media/video
content where files can be very large and thus more time consuming to
download.

A practical instance of when you might want to merge two mirrors is if you
have a "full site" mirror of a dynamic site and you know for a fact that only
a small section of that site changed. There is no good way to update just
*that* small part without being forced to download the entire rest of the site
unless you create a separate mirroring project for that part of the site.

I just wanted to point out that there *are* practical situations where this
functionality would be useful, and it's not just some bizarre edge case I made
up just because I like to complain or something ;)