If you’re running 6.1, there is apparently a fix for it. From the read me for TNC_6-1_Fix4:
On the Processing Rules screen, when the user adds a partner profile for senders or receivers, Trading Networks removes the existing profiles.
The Trading Networks Console allows you to specify a list of senders and receivers on both of the following tabs:
Previously, when you selected to add partner profiles to this list of senders and receivers, Trading Networks did not append the new profiles to the existing profiles.
With this fix applied, when you add profiles to the list for senders and receivers, Trading Networks appends the new profiles to the end of the list. This service pack adds the capability to delete from this list as well.