Okay, this is what I did:
I created a fully hosted subdomain using the same user as domain.com. The document root was set to /home/username/xxx.domain.com (this was the default value, so I did not have to change this). I already had the files for both subdomains at these locations, and Dreamhost did not delete the files upon creating the fully hosted subdomains.
Next, I edited each subdomain after creation and selected Create a new user from the drop down list. I was given the option that the files would automatically be moved to this new user (checked by default, so you do not need to do anything here). I entered in the username I wanted, and submitted my modified changes.
Then I changed the passwords of these two new FTP user accounts to the ones I had on SimpleCDN.
And finally, I plopped in an .htaccess with the following contents into the document root of both subdomains (so that directory listings would not be generated):
allow from all
# Prevent access to HTAccess
deny from all