When encountering a 403 Forbidden error while attempting to join tables across two Unity Catalog instances, the root cause is often related to catalog-to-workspace binding constraints: by default, a catalog is only accessible from workspaces explicitly bound to it, regardless of any GRANT permissions your user may hold. To resolve this, ensure both Unity Catalogs are properly bound to the same workspace—or operate within separate workspaces where each catalog is assigned—and that you have the required USE CATALOG, USE SCHEMA, and object-level permissions in place. Once both catalogs are accessible within the same execution context, your cross-catalog joins should succeed without permission issues.