There is a use case for having all ideas in a workspace belong to a primary portal ("parent" portal) as well as a secondary portal.
To achieve this today, all portals must be set to restricted, and bulk updates made as new ideas are submitted.
This is because new ideas will only appear in the restricted portal they were submitted in by default.
Users will need to periodically perform bulk updates to bring the ideas into the primary portal + secondary portal
Add an idea automation rule that triggers assignment of ideas to the specified portals