According to the latest release notes we can now add up to three FTP users per MID:
This release allows you to add up to three FTP accounts per MID. Create or delete these users in the Setup section of your account on your own. Restrict account access by specifying the IP addresses a user can access the account from. This new feature gives you the flexibility to assign different users to different business units as needed.
https://help.salesforce.com/articleView?id=mc_rn_january_2020_platform_ftp_users.htm&type=5
But anytime I try to add new users the username is locked as the MID and it claims that this user already exists. Is there something I'm missing?