I am not quite sure what you mean. The registration function in the Web Panel is limited to one domain at a time, but you can register two domains one after the other if you wish. The 'window' for someone else to take your domain would be very small.
During sign-up you can register one foobar.com (or .org or whatever) as part of your plan cost, registration of this domain will remain 'free' for the life of your hosting at DreamHost.
You can also choose to not register this free domain during the sign-up process and instead choose a subdomain of dreamhosters.com. In this case, your free domain can be registered at a later time using the Web Panel.
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