Configuring PureFTPd server for double logon

I am using PureFTPd as FTP server on Mac OSX. I want to configure the PureFTPd server with double logon feature for security reasons. Can you please guide me how I can achieve this? Here is what I want:

1. Users supply username/password for logging in to FTP server
2. Users specify another set of username/password to go to the directory

Can this be done within PureFTPd, or do I need some external application along with PureFTPd, i.e. some firewall? If you can direct me to any resources on the Internet, it will also be very helpful. If I need to use some other FTP server, I can also look into this.

Thanks for your help.