When nobody crosses the finish line by the end, the winner is the person who has lost the most weight, as a percentage of their total goal, by the final weigh-in. For example, suppose the challenge is to lose 4% in 4 weeks and the two frontrunners are Joe and Sally. Joe had to lose 10 pounds to reach the finish line. Sally had to lose 6 pounds to reach the finish line. At the final weigh-in, Joe had lost 5 pounds, or half his total goal. He's at 50%. Sally had lost 4 pounds, 67% of her total goal. Sally wins.
If nobody loses any weight, or everyone fails to submit a final weight, then no one splits the pot.