There is a plugin that claims to do (we haven't tried it ourselves) what you are looking for.
It is called Domain Mapping System. From the plugin's description:
"Domain Mapping System allows to map domains to WP pages and/or custom types."
"...Imagine you've got a company website based on wordpress with the domain www.mycompany.com, all good so far. Furthermore, you've got two products with own domains, e.g. www.myregularproduct.com and www.myspecialproduct.com...."
In case you haven't figured a way yet, give it a try and let us know how it goes.