Are people still allowed to log into the original arena?
As I said if a 400 player arena splits in half the result will be 2x200 player arenas that both can take new players with ease. Or switch sides.
As long as neither server is full and neither server is practically emty players should reasonably thinking be satisfied.
Then if numbers drop to 2x80 or something, more and more players will start to pack the larger server untill smaller dies off and the larger at some point grows back to split status.
You could even implement a minimum threshold that would boot the players on the 'losing' server to the larger one if the larger one has less than optimal amount of players.