You can put the two objects in a Pair family and add an instance variable to this family, like "pairUID", in wich you store the paired object UID: when you pair object A to object B, set A.pairUID to B.UID and B.pairUID to A.UID.
Then, when you need to pick the two objects, just do use two conditions in a single event:
- A condition that pick object A
- B.pairUID = A.UID