Combining multiple MyCloud (1 EX, 1 PR) NAS + other drives on a Server that uses Filezilla

Why don’t you use the DNS and FTP server of the PR4100?

Via SSH you can mount the EX4100 onto mount point inside a share of the PR4100, so your clients can access all data via a single FTP share.

For filezilla server support, you may have to look in the filezilla forums.