possible to have 2 Time machine "silos" on same machine?

[guitarist]

I think your prerequisite that the data be stored on separate physical backup disks might be making it unnecessarily complicated. If you were storing the data in separate silos (folders, directories, whatever) on the same external drive would it work?

Or is the simple fact of trying to instruct Time Machine to backup a boot drive AND a second drive causing the barrier to doing what you want?