Offer a Couples DietBet Game
This would be a pair's only dietbet. Each person weighs in and has their own amount to lose but both people in the couple (significant other's, friends, whatever) have to reach his/her goal in order for them to win as a team.
This would be more motivating for each person because no one wants to be responsible for making the team lose.
L O V E this idea!!! P L E A S E !!! Take some of YOUR winnings and ask your programmers to GET BUSY! You could have a tab "More complicated options for groups!!!"
Yes, you could have this as an option on any diet bet, wouldn't have to be a separate "pair's only" diet bet, and you wouldn't have to limit it to pairs. 2 people (or 3 or 4 ie those competing with office friends) could sign up in the same diet bet and then link themselves such that unless everyone wins, nobody wins.
Toby Delrahim commented
This would be cool. I am doing a buddy challenge at work and would have liked the ability to sign up for a related dietbet during that challenge..
There could also be an additional incentive. Like each player is responsible for their individual game and individual bet, but the couple (married or friends, maybe even a small group?) could make an additional bet together that they only win if they both win. Kind of like doubling down in poker.