Re-connecting to a moved Time Machine backup

I recently purchased a new Retina MacBook Pro (herafter "2012 MBP") to replace my stalwart but creaky Late 2006 MBP (core 2 Duo).

On the 2006 MBP, I had an external drive connected via FW800 that held my Time Machine backups since Leopard (it now runs Lion). Backups were running fine until I split it all up, and re-configured around the 2012 MBP. That same drive is now connected to my 2012 MBP via an Apple Thunderbolt to FW800 adapter. I started Time Machine backing up to this external with Mountian Lion, and that is working fine.

What I would like to have happen, is the 2006 MBP continue to use the same Time Machine backup, only this time via wireless networking to the 2012 MBP. I have seen several articles on the net about setting up a Time Machine backup on a shared drive on a remote Mac. All of that seems to be working fine. From the 2006 MBP, I can enter Time Machine Preferences, and see the remote drive. I can even tell it to start backing up. But my problem is, it seems to be starting a new backup, rather than connecting to the existing one, and Time Machine offers me no way to drill down beyond the shared disk.

On the drive - which has only Time Machine backups, I have one folder: "Backups.backupdb" (or some such), and inside that folder are two folders, each of which has the name of the MBP that was backed up to it. These appear as folders in Mountain Lion, not any other kind of file. When I tried to re-connect the 2006 MBP through the 2012 MBP, and start the backup, it creates a new file at the root with a "sparsebundle" extension.

Maybe I forgot something, or maybe the "sparsebundle" is just a distraction, but Time Machine on the 2006 seems to report the "last backup" as "never"...

I suppose it wouldn't be too difficult to just move the cable when I need to back up the 2006 MBP. But I'd rather leave it connected to the 2012!

Thanks,