It is difficult to give further instructions before you first familiarize yourself with what I explained earlier.
Perhaps you could build a small test library. For instance:
1) On the downstairs computer create a new local folder, e.g. "C:\shared" and copy a couple of small local audio and video folders to it. Share this folder as DS-shared
2) Do the same on the upstairs computer. Share this folder as US-shared.
3) Still on the upstairs computer:
- map a drive letter to both shared locations. E.g. map X: to \\[dowstairs computer]\DS-shared and Y: to \\[upstairs computer]\US-shared
- create a new library, load it and set Auto-Import to monitor the X: and Y: drives.
- create a library backup file
4) Back on the downstairs computer:
- Replicate the drive mappings exactly as you did on the upstairs computer ( X: to \\[dowstairs computer]\DS-shared and Y: to \\[upstairs computer]\US-shared ).
- Create a new library.
- Restore the library backup file you created on the upstairs computer.
You have now identical libraries on both PCs. The libraries see the same files through virtual paths. The local files were not physically moved on either of the PCs.
If you close one of the computers you can still access the files that are local on the other PC. The files that are on the closed PC will be offline, but preserved in the library if you don't set Auto-Import to "fix broken links".
Also while I'm going through this I will obviously be making changes to the library as it relates to the file path. I have a lot of home videos which takes a great deal of time to get thumbnails for (m2t) so is there a way to point the existing files to the new path rather than removing them from the library then reimporting them?
You can use the Rename, Move, & Copy Files tool also for just changing the database links. It preserves the existing thumbnails. I have posted some instructions here:
http://yabb.jriver.com/interact/index.php?topic=55224.msg375699#msg375699