Requirement:  2 unqiue users from a Role need to approve for the activity to succeed.    Succeeding rule specifies at least 2 slots need to approve.

#1 Plan per destination:  Plan Just Once

#2 Specify the number of slots to be created: 2

#3 Create a slot for each role and group.  (I guess this is N/A when Plan Just Once is used)

The destination is assigned to a role and there is only 1 role.

Edit:   The problem is that the same user is allowed to approve twice (is that expected?) completing the request.    Do you put the restriction logic in the UI to prevent the same user from approving twice?

http://help.k2.com/onlinehelp/k2blackpearl/userguide/4.6.8/webframe.html#Destination_Rule-Destination_Rule_Options_uContinued].html

Plan Just Once

Because only one Activity Instance Destination exists, the Client Event will only execute once. There will only be one Worklist header and one Serial number. So all destinations will have access to the same serial number (Activity Instance Destination). Each response from the user will update the Activity Instance Destination XML/data fields. A slot will be created for that user and the XML/data field values will be copied to the slot and depending on the action performed, the slot will be completed, or remain Open to the user.

The activity instance will complete when all slots are completed or when the succeeding rule is true.