Chiippie has already said something about it:
"Matches are usually played in CTF. This means, if one player wants CTF and the other wants Battle, CTF is the mode players will play in. On the other hand, if both players prefer Battle, Battle will be played. About the levels: this is also arranged by the players themselves. If they can't get a real compromis about which map they'll play on, they should contact me. In that case I'll pick a map from the JJ2WC Mappool (in case of CTF) or a random Battle level. But this is not likely to happen."

;p