SQL Server : Execute as multiple users simultaneously? I need to query a proprietary database where the software vendor has set tight permissions on each schema (one username per schema), and each schema-user is only allowed to read from their own schema.

I now need to run a query across multiple schemas. Is it possible to run a single query against multiple user accounts? Something like EXEC AS USER 'A' AND 'B'?

Obviously the correct method is to create a user with all required permissions, but I am running the query from a stand-alone utility and will not have admin privileges on my future clients' instances.
No, you can't.

You could create user C that has permissions of A and B...