I believe you are referring to extra library which is managed by CDL for staging/cloning data via embedded NW SN. I never worked with that so I can't say how this is presented to NW server. If it is in that case you can share it. If however this is not presented but rather configured inside the CDL box only in that case you can't share it.

Please let me fine tune the question: can I share a tape drive between NW server and CDL as long as I have a DDS license?

CDL has its own drives & server would have its own...what do you want to share?

DDS license is per "physical" drive presented to NW.