Thinking about the apparent impossibility of the top scoring couple to be in the dance off.
I suppose if the scoring is done by giving 1 point to the lowest scoring judges couple and adding one for each place higher on the leader board. And then repeating the process for the public vote, and then adding together the placing marks....
In theory, the lowest possible score for the 1st judges placed couple and the highest possible score for the lowest placed judges couple would be the same. And indeed, everyone could score the same total points.
Say there was 10 couples to start with,
Couple Judges. Public. Total
A. 10. 1. 11
B. 9. 2. 11
....
I. 2. 9 11
J. 1. 10. 11
I would assume the bottom 2 would be selected by their position on the original judges leaderboard.