Hmm I am lost now. If you already have 'seconduser' setup, just go to the control panel and choose 'Domains > Web' and edit 'domain.com' so that the user is 'seconduser'. If you want http://domain.com/ to show files from '/home/seconduser/domain.com' make sure Web Directory reads as '/home/seconduser/domain.com' otherwise make it read as '/home/seconduser/seconduser.domain.com'. Then login to FTP as seconduser and upload the files to the appropiate directory.
Perhaps I got confused by what you mean 'main directory'. I guess you mean home directory. A home directory is the top-most directory for a user, and is written out as '/home/user'. Domains or subdomains will appear as subdirectories of a users home directory and neither a domain or subdomain subdirectories can be called 'main directory' as the heirarchy of domains usually only applies to DNS and networking and not the file system. For all Apache cares the pages for http://dreamhost.com/ could be served from a directory named 'the-best-host-in-the-world'. However when logging into regular FTP DreamHost has it setup so you can't go above '/home/user' to '/home' or '/' so their FTP software will show '/' when it really means '/home/user'. I originally thought you wanted to share access to your home directory or subdirectories with another person. As will said, thats not as easy as one would like.
Perl / MySQL / HTML+CSS