If you are shopping for an archival system, you shouldn't be worried about how to backup the archive. Instead, you should look for a system that has the appropriate level of protection for the data placed into it. By nature, both an archive and a backup are extra copies of data 'just in case'. A backup is an extra copy of data 'just in case' something happens to my primary copy of data. An archive is an extra copy of data that I no longer need 'just in case' I find out later that I do still need it. As such, a backup of an archive is an unnecessary excess. An archive of a backup, however, is an understandable archive albeit a poor one.
Instead of looking for ways to backup archives, you should again ask the question, why. After you know why you are making an archive, you can determine what your requirements are. With those requirements, you will find that backups are not included. To protect and against component failure, you need redundant components. To protect against modification, you need WORM. To protect against corruption, you need data validation. To protect against total site failure, you need replication.
Although a backup system may have similar characteristics, it is built to protect live data which has its own concerns. Live data is just that--live. It is changing and active. You can't just lock it down to protect it. Users are constantly making changes do it. Some of those changes may be in error. Data will need to be recovered from those errors. Archives have the luxury of protecting inactive data. This means that the data is not changing. It is static by nature reducing the scenarios which could cause risk to the data also reducing the complexity of the system needed to protect it.