I think what we're looking at here is a sort of “requiring item” (sorry but I can't find the correct term for this) where unit 2 (the CD in this example) may only be issued if its requirements are met - that unit 1 is already issued to the borrower, and similarly on return. I would need to add an item field and linked MARC subfield to Koha so a cataloguer can set this as a reason not to issue an item but I don't see any particular problem with that. http://wiki.koha-community.org/wiki/Requiring_items_RFC
This is still valid.
I wonder if we could mark this resolved now that we have item bundles as an option to handle this use case?