Sorry, but you can't.
Each user gets their own home directory. FTP users are restricted to their own home directory. Since you said "FTP" this is why you cannot have a second user use FTP to access another users home directory.
Each domain is hosted from a subdirectory of a particular user. You can set the second domain in the home directory of the second user, and if you need to share management, share the credentials of the second user.
SFTP/shell users are not restricted to their home directory. If you are comfortable with it, you can attempt to share management between separate users with UNIX permissions.