Expanding a bit this would be great in an IDE, Often I have wanted to simply select a bunch of files and edit them at once (maybe with a special temporary comment between them) as a single file then save the changes back to the individual files. Would be nice if that was a right click context menu item or bindable command.
In C(++) land I could see it being extremely useful for h/c(pp) juggling if the IDE automagically did the combining.
Finally being able to write the comments that separate the files as I type would be really nice when prototyping out new code you want all together before you split it up.
Also perhaps if its common enough certain meta files that describe link other files together could be left as shortcuts in the repo. When you open those the files (or portions of files) they reference are opened for editing.