Possible to backup External Disk to Time Capsule regardless which machine it's connected to? Here's the scenario: I have two MacBook Pros (Foo and Bar), and their drive's are both called Macintosh HD. I have a Time Capsule called Time Capsule, which is used to back up both Foo and Bar. I also have an external disk called Stuff which I connect to both Foo and Bar.

Is it possible to have Stuff backed up to Time Capsule in such a way that regardless of whether Stuff is connected to either Foo or Bar, the same destination on Time Capsule is used? I.e. can an external disk connected to two different computers be backed up to the same Time Machine disk without duplication?

Seems like this might be possible with tmutil but I haven't been able to figure it out.
well good question and well described. It is possible to BackUp 2 Macbooks Fooand Barto one TimeMachine Disk. 

I had some problems with SnowLeopard but with Mountain Lion it was a lot better and now with Mavericks it is working fine. 

So if you have Mavericks installed on both it is no hassle so far in my Envirmonent (have 4 Macbooks and one TimeMachine Disk (in the Network))

Example of may TM Diks

TimeMachine
|- Macbook Pro Martin
|- iMac Martin
|- MacPro Martin
|- Macbook Air 


 Folder has been renamed in this example 

So the Devices are seperated in Folders on your TM Disk.