There are no rules in EXTREME NAUGHTS AND CROSSES. Play against a friend, in teams, whatever you like. One thing to note the first piece on each board is a cross.
Here are some play suggestions...
Game Style 1: Place as many boards as you like. Then each player must place a piece on each board (but not in the same place, unless you play over 9 boards) before moving to the next player.
Game Style 2 (True EXTREME NAUGHTS AND CROSSES): Place 1 Board. At any time a player can place a new board and take the first turn on that one. Each 'go' you can only place one piece at a time, no matter how many boards there are. Remember, the first go on a new board is a cross. So if naughts starts a second (or third..into infinity) board they play as crosses on the new board.