If you have this facility in your hosting package, then you can manage different FTP accounts from your control panel. You can add FTP accounts allowing full access to your directories, or restrict access to a specific subdomain (also configured in cPanel for that account).
Note that any additional FTP account configured for access to a subdomain will have slightly different authentication data.
Also, the domain must be used with the following FTP authentication details, having the following format:
user@domain.com (user = the user configured in cPanel when you added the FTP account)
domain.com = your domain name.