Sure. If you created a FTP for one website, you had to create a user to login to and be able to do stuff with your FTP account (like read, write, execute).
Now if you made it work for one user, let's call him larry. You would login as larry and use his password. That way you would come to larry's home directory, presumably /home/larry. You can set a users home directory in the /etc/passwd file or with usermod command (see man usermod for details on how to use the command).
Now, if you have a FTP account for one user, just add a new user, using the command useradd or adduser new_username(whichever works on your system). When you add the user (if it doesn't ask you for a password), assign the user a password, using the passwd command, like this: passwd new_username, where new_username is the name of the new user you just created. Let's call her betty.
The commands would be:
adduser betty or useradd betty
Now you have the new user betty and you have a password for the user.
This is pretty much it. If you'd like the user to be able to write only to a chosen directory, you can assign that directory as a home directory to that user with the usermod command, or you can set a link to the directory in the user's home directory.
Wit this information you should be able to make it work, or at least set you in the right direction.
The best place for more information is your FTP server's website. You say you use plesk? I never heard of plesk, but it should be the same in most of the servers.
If you run in any more problems, post them here, or you can msg directly, as I don't look at LQ every day.