With the default providers, my answer is no, you cannot use a single role provider to manage users from two membership providers. That is because of the database structure, used with both Sitefinity default providers and ASP.net Sql membership/role providers:
There are three tables in the database related to membership : telerik_Users
You can see the relation between users and roles is "many to many".
So, when for instance the GetRolesForUser("John") is called, the provider looks up John's ID in the Users
table, then checks the corresponding role IDs in the intermediate table and finally checks the role names in the Roles
table. This is pretty much the same in the default sql implementation of the ASP.net providers.
However, with custom providers you can manage multiple membership sources with a single role provider. Of course, you'd have to be very careful with duplicating user names/emails, and synchronization of data. As you probably noticed, user name is considered unique in the role provider's methods so this won't be an easy task.
All the best,
the Telerik team