I have several contacts (and in fact I do myself) that have more than one business. I have each of their companies on the database, but I can only link the contact to ONE company. Does it work to have multiple entries for the same contact on the contact list, linking each one to one of the companies? The reason is to keep call notes separate for each business, depending on what 'hat' the contact is wearing when I call.
Or you can associate each Activity/Note/History with the correct company
You can also add a contact to multiple companies, but only link them to one - from the contact, Groups/Companies tab, change Show Membership to Companies and click Add Contact to Company