UNIX (DreamHost's operating system design) doesn't readily allow for people to co-own directories and files. If you want to completely co-own your website, you'll have to use the same FTP username and password.
What you're seeing is that each person has their own home directory. FTP won't let a user go to another user's home directory. However, SFTP will, but there's still the ownership issue.
I see two options to completely share your home directory:
1) The easy way is to use the same username and password. I always create an FTP user for each domain I create. I suggest you create a new user you can both log in as and put your website there (i.e. go to Manage Domains and change the owner).
2) The hard way is to set up a cronjob to constantly assign group write privileges to all of your website files, and use SFTP so that user can switch to your domain directory.
Oh, and there's no public_html folder. Inside your directory with maildir and logs is YOURDOMAIN.com, the "public_html" directory for your site.