Each of the two players are simultaneously master and student. Each player comes up with a rule and builds one koan that has the Buddha nature and one that does not. The first player marks his koans with white and black stones and the second player marks her koans with green and red stones (white stoned koans have the Buddha nature for the first player's rule and green stones have the Buddha nature for the second player's rule). As such, each koan on the table is marked with two stones, meaning it can be white-green, white-red, black- green, or black-red. Play begins with a randomly chosen player, and each player takes turns building a koan and calling master or mondo as per normal. With the exception of the starting koans, you do not mark the koans you build until after the student builds a koan for the round, but before the master/mondo call. Play continues after a player correctly guesses until his own rule is also guessed, or the players agree to conclude.
If you wanted to inject points into it, I'd say to award 1 point for each correct guess, and a bonus point if you guessed correctly on the turn immediately following the other player's correct guess, or perhaps you could use Kory's range idea in conjunction with this.