We need to be able to connect both Salesforce and Zendesk into one idea portal. Salesforce requires SSO. Zendesk does not. Our support team is in Zendesk. Our sales team is in Salesforce. Is there documentation that demonstrates how to glue both of these worlds together?
I recommend that you use separate portals for Salesforce and Zendesk integrations.
However, you can have the same ideas appear in multiple portals by choosing the same products in each portal. Then the ideas created via Zendesk will be visible to Salesforce users and vice versa.
You could also create a third public or private portal to also provide access to the same ideas.