Winning and Losing
The goal of these debates is not that one person win and one person lose. Rather (as stated before) it's to inform and make the strengths and weaknesses of each side clear to all observers and participants.
However, it is of course possible for one side to emerge with a clear advantage.
So at any point in the debate, the referees can each state their vote for who has the "advantage" at that point in the debate (Pro, Con, or Neither).