Can not add both Individual and Organisation to case
I too had this problem. What worked for me is modifying the Contact Type A and Contact Type B to All Contacts in the Relationship section (administer---customise data and screens---relationship types) Remember to do it for any Case Related Roles e.g. the Case Manager. I was then able to select the organisation type. I have set it up so a User can submit the case as the Original Submitter but the case is assigned to the Organisation so multiple users can submit cases but they get organised to the Same Organisation which makes it easy to keep track. Hope this helps.