You got it. Once the intended quarterly jobs are copied from the existing Aux Copy to the new one, you can likely disable the Extended Retention for those intended fulls (but if you don't ened the tapes back yet, just let them age on their own).
You can absolutelty use the same library twice, so that's not the issue. I'm wondering if you only have 1 Tape Drive (so you can't copy from 1 to another)?
Is there more to the error?
Tell me about the Primary Copy itself and the existing Aux Copy.
What library do each use? Are the Deduplicated? If so, is the Storage Policy using a Global Dedupe policy?