Sorry, Andrea. I don't mean to be a party pooper but competition is not good for me in weight loss. I, personally, just need to chug along on my own. Maybe someone else will take you up on your challenge.
It is not a fair thing to do if you are going to judge by pounds. The only way it could be fair is by % of total weight lost. I don't think these challenges are beneficial. How do you plan to measure the success of core vs flex?