I am assuming you have some kind of object model which has a 'list container' with relations to the 2 lists (since you have a form where you can select these 2 objects)
Going by that scenario, try this:
On the reference selector for the list2 object, open the property 'Constrained by' (under Behavior - advanced) This should show you only 1 path which you can enable to constrain this reference selector by the object selected in the list1 reference selector.