Yes! I'm not sure why the Nathan is suggesting you set up another plan. Each DH webhosting plan allows you to host as many domains as you want under that single plan at no additional charge.
To set up your second domain, just log into the Control Panel-->Domains-->Manage Domains, and select the "Add a Domain /sub-domain" link at the bottom of the page below your previously set-up domains.
Fill out the screen that appears, and you are good to go once everything updates.