Dear Adriana,
We are in exactly the same situation: Contacts are shared between units, with each unit sending its own newsletter. Contacts should be able to unsubscribe from one newsletter without being unsubscribed from other newsletters. In that situation you should not (never) allow recipients of your mailings to perform a global unsubscribe. Instead, work with Subscription Lists per newsletter, create a Subscription Page per newsletter, and insert the link to the corresponding Subscription Page in each newsletter template. That's how we do it.
Good luck!