I've been able to set up multiple application services for one RIA Services app, while each application services have its own user definition and rules for authentication. I've heard customers wanting this kind of configuration for their sites, for example, a standard user authentication mechanism co-exists in their app with another one that's for VIP users only, with different URLs as the entry portal.
Here is what I have done, it is really simple to set up.
1. Create a new Silveright application with RIA Services enabled or create a new Silverlight Business application.
2. At the server, define multiple user and authentication service classes that either derive from the UserBase class/AuthenticationBase class or implement the IUser/IAuthenticationService interface.
3. Modify the init parameter in the .aspx page of the server project.
4. In app.xaml.cs file of the client project, associate different web URLs/init parameter with authentication service that's defined on the server.