One possible approach (not necessarily the most efficient one) would be
• storing the first Selection
• creating the second Selection
• storing the second Selection and the added Selections
• creating Working Paths from the (expanded) second Selection and the added first and second Selections
• checking which of the two Paths’ subPathItems have identical points
• creating an inverted Selection of that subPathItem and filling it in the added Channel with black
• loading that as a Selection and removing the Channels and Paths
Edit: But I guess this would mean that one has store the first selection, create the second and then start the Script.