Possible to backup External Disk to Time Capsule regardless which machine it's connected to?
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.