Sure, there are different types of users on a hosting system. You are already familiar with the panel user. What I am referring to is the user account on the web server. That user goes by many different names (Web user, FTP user, Shell user) Basically it is the user that you would log into if you wanted to upload, edit, or otherwise add files to your domain. The one click install can create a new user account for each forum, or you can manage both domains under one user. While using the same user for both domains can be convenient, having separate accounts with separate passwords per domain is better from a security standpoint. So, if someone manages to exploit domain-a.com, they shouldn't automatically have access to domain-b.com (Although an attacker could still exploit each domain independently). This is also better from a organizational standpoint.
You control which user a domain is run by visiting https://panel.dreamhost.com/index.cgi?tree=domain.manage& and clicking edit under the hosting column. Then change the "Run this domain under the user" field, if needed and clicking the change settings button. Wait about 5 minutes to be safe and then proceed with the advanced one click install one domain at a time.
The same rules apply for the MySQL users. A MySQL user if different from a web user. MySQL is a database that contains the backed data for for your forums (threads, posts, forum users, etc) Just keep the configure MySQL database automatically box checked. It will handle this for you.
I hope this helped to clear up your confusion. If you have any other questions, let me know