Feature Req: Sync changes to different media

[mobilemcclintic]

This has been brought up for uses like web site updates (SVN/Git versioning), but my request is a little different.
I have remote sites with slow enough internet or bandwidth limits that it is feasible to scan for differences, but not to actually sync them.
I would like to be able to compare two folders/groups of folders across a vpn network share, and sync the "new" and "replace" files to an external drive so that I may physically go to the other locations with portable media that doesn't have to hold the complete folder set.
For example, I have backups of all of my families PCs including documents and photos that are irreplaceable. If I could compare the Site A and Site B (C, etc) from Site A and copy the new files from Site A to a hard drive to be taken to Site B (C, etc) (I routinely make family visits), then sync the changes to Site B, and do the same thing in reverse for B to A, it would be a life saver.