Yes it's possible to run multiple sites on one db. The key is in host header mappings. Once you have created your new site using the administration > site settings menu, you will see a tab to set that new site's host header. Basically when you are done, you have multiple DNS entries all hitting your web server, which looks up which site to serve based on the host requested. For example you could have www.sitename.com and www.totallydifferentdomain.com both pointing to the same mojoportal server and serve two completely separate sites, all from a common db.

Below are a few details of the process. It's worth giving a try to see how it works, you can delete unwanted sites later if you like.

Go to Administration > Site Settings. Drop down the select box next to Site Title to select "New Site". Give the new site a name. Choose site slogan, company name, email address, and time zone settings and hit SAVE. The site is now created.
- After you have saved the new site, you will see the tab in Site Settings for Host Name Mapping. Make sure you are still on the new site that you just named, and add a new host name mapping for that site by typing the full host name (ie subsite.yourdomain.com or www.newdomain.com) and hit "Add Host Name" button, then hit "Save". This should match the DNS settings for that URL. The new host name must also be added in IIS (or your particular web server) on our server it's Edit Bindings (IIS 7.x) for the Site.
- Once the dns has propagated (or you hack your local hosts file), you can browse to the new site.

There are many different options related to multiple sites, I encourage you to review the documentation here in detail before you go too far down the road.