We need this feature also!!!!
On new client implementations, there are usually two account managers involved and only one can see the ticket in their my incidents in the customer portal. The other one keeps opening new tickets to get updates. We added the 2nd person to the watchlist, but they only get copied on updates if we remember to check the box and do not have an easy way to see the entire ticket/history in the customer portal.
The external watchlist as a work around is inadequate. There is no access to the customer database, you need to manually enter an email address even if the customer already is configured in the system. Finally EXTERNAL watch list is explicit...they are EXTERNAL whereas we want to be able to associate more than one CUSTOMER to the ticket and update all associated customers when we send notifications to customers.
This issue has been around a long time judging by the message I am responding to. It really deserves to be addressed.
Perhaps you only allow linking to one customer but under the Customer Notification area you can add additional customers from same company to be notified?