Bergans of Norway 2095 Alpinist 130L backpack with detachable second backpack

Bergans of Norway 2095 Alpinist 130L backpack with a detachable smaller second backpack. Very durable, rugged packs but we have not been able to figure out exactly how the second pack should be attached since my husband has never used that one. Used condition, normal minor wear and tear but everything is functional.