You can add as many sites as you want to a single hosting plan, but be sure you buy a plan that's sufficient to accommodate your expected needs.
A DreamHost plan will include one free domain registration. You'll have to pay for the second (unless it's already registered somewhere else). Once you set up your account, you can easily add your domains and include hosting for each.
The control panel is pretty self-explanatory on this and the help Wiki contains the rest of the material.
Co-Host, The Paracast
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