Under OS/2 there existed the "Toronto Virtual File System", which allowed multiple directories to be combined into a singe directory. When you accessed an existing file, the virtual file system (VFS) would know where to look for the real file. If you created a new file, it would create that file in one of the specified directories that was designated.
This allowed for combining a directory on HD and one on CD, where all access would go to the CD, unless the accessed file was also on HD or it was changed, then the HD would be used.
I always liked that idea and wish something like this existed under Windows or Linux, but I could not find anything like it. Can anybody help?