Short answer
The main risk is choking, not poisoning. Keep the other dog away from any remaining octopus and monitor the one that ate it for gagging or trouble swallowing. If you see any signs of distress, call your vet right away. Otherwise, just keep an eye on both pets and don't offer them any more.
Octopus — Food safety facts
Feed with care · Dogs · full breakdown on the linked food page
Very rubbery and a genuine choking risk even when cooked. Best avoided.
The full verdict, portion guidance and serving notes are collected on the Octopus food facts page.