Is it possible to create a distributed collaboration between ArcGIS Online organizations?

Answer

No. It is currently not possible to create a distributed collaboration between two ArcGIS Online organizations. According to ArcGIS Online: Considerations for creating a collaboration, only one ArcGIS Online organization can participate in the collaboration. This means collaboration between an ArcGIS Enterprise-to-ArcGIS Enterprise organization or an ArcGIS Enterprise-to-ArcGIS Online organization is supported.

As a workaround, contents can be shared between ArcGIS Online organizations by creating a shared update group. This group enables all group members to edit shared items and limits the group membership to the members in the organization. Refer to ArcGIS Online: Shared update groups for more information.