Tape backup software for putting multiple backup sets on one tape [closed]

Check Veeam Backup and Replication. In v9.5 they allow to append incomplete tapes. https://helpcenter.veeam.com/archive/backup/95/vsphere/add_gfs_media_pool_advanced.html

The community edition is free for a use of 10 or fewer VMs. Haven't yet upgraded to their new v10.

Bacula and Bareos does this well. They are able to write to the tape until it is full, then they ask for another tape. AFAIK they are Linux only, except that they do have a Windows client agent for backing up Windows pc-s, but the central server(s) must run Linux.

However, according to my experience these are complex and somewhat fragile softwares. Considering that you already have the data to backup on one machine via ftp, first I would just try a shell script from a cron job which append one or more tar files to the tape and email you every week to change the tape.