No, not all members of organizations with a partnered collaboration can edit a dashboard owned by an organization in ArcGIS Dashboards.
A partnered collaboration enables two involved organizations to collaborate on data collection in a cross-organizational sharing group. This is achieved through concurrent contributions of information on a hosted feature layer with the enabled-editing setting. However, not all members of the partnered collaboration can edit the dashboard owned by the organization.
Only organization members with sufficient privileges can edit or update the dashboard in ArcGIS Dashboards.