What if you have seperate logins for each admin with their own corresponding home directory being that of the root of their respective web site? I don't know if that would work as I've never done it before but I'll try it when I get home

In Internet Service Manager, open the FTP Service Properties.
Select Directories.
Select the directory that will be the home directory for the FTP service.
Click Edit Properties.
Enable the Home Directory button, then click OK. Click Apply, then click OK.