From the FAQ: Insufficient material remains on the board to finish the game. If only two kings remain then either player may claim a draw during their next move. If you Google it, two remaining Kings represents a 'tied' game, but is not stalemate. I guess the game could continue infinitum, but then would fall foul of the No piece capture or pawn movement has occurred for over 50 consecutive moves (100 half moves). Therefore the game has not progressed and a draw may be claimed by either player during their next move rule. Either way, a draw has to be claimed it seems. There, I've learnt something today......!