Are multiple purchasing category hierarchies possible

Summary

In an enterprise with two ledgers and one BU per ledger is it possible to create a different set of purchasing categories, along with separate hierarchies, for each?

Content

In an enterprise with two ledgers and one BU per ledger is it possible to create a different set of purchasing categories, along with separate hierarchies, for each?  As far as I can see you can just define one set of categories in the Manage Functional Area catalogs task and they cant be assigned to a particular Procurement BU.  Dos this mean it needs to be shared across the enterprise?