I haven't tested this (and so I may be wrong...) but here is my thought if I understood the question correctly --

You have a stored procedure in schema2 (user2) of db2
This procedures touches some objects in schema1 (user1) of a remote database (db1)

1. You should grant appropriate permissions (execute etc.) to the user1 in db1.
2. From db2, you should create a db_link with fixed user(user1) to db1.

Let me know how it goes.
Thanks,