I have been trying to only use encrypted sparse bundles as my targets for Time Machine. The benefit of doing this is that these are easy to move around, as compared to the traditional Backups.backupdb folder.
So here is what I did. On Mac X I created a sparsebundle (call it TM.sparsebundle) on an external disk. Then I set it as my target for Time Machine (to do this see How can I make my computer believe a disk image is a hardware disk?) and backed up to it. Then I disconnected the external disk and connected it to Mac Y. What I want to do is to use Mac X to backup to TM.sparsebundle which is now connected to Mac Y. I can get it to do this by mounting the TM.sparsebundle on Mac X and then setting it as a TM target (using the same method as before).
But here is the problem - I can't get Mac X's TM to automatically connect to Mac Y and mount the TM.sparsebundle. Everything works if I do the connecting/mounting manually.
Of course if I had initially created the sparsebundle from Mac X while being connect to Mac Y then it would have worked. So the issue is how to get the TM running on Mac X to treat an existing sparsebundle on Mac Y as a legitimate, fully supported TM target and auto-mount it.
If you are thinking this is answered by the Great Pondini, it is not - at least not exactly. He has many variations on this theme (http://www.baligu.com/pondini/TM/18.html) but does not address this particular situation. I am going to try to adapt one of these methods but it is very slow to even test this so I am hoping someone here will have a definitive solution.