Wait, what exactly are you trying to do?
-If your looking for someone to challenge I would suggest going through the music contest and finding someone that you'd like to challenge (challenge someone you think will be interesting, not just someone you know you can beat). Post challenges that you want to make in the challenge submission section - this would be the first Music Challenge so thats always good..
-Are you asking for two people to challenge each other on that theme which you will then use for your map which ever one wins? If so then this is more fitting to just ask some people to make music as a sort of mini undeclared contest - If this is what your trying to do then your better off just doing as I've said and making a personal sort of mini challenge as it pisses people off when people try to act like their so smart in devising a way to get people to fight over something that you inevitably reap the benefits (Kinda like trying to start a contest to get other people to finish your own map for you).