There's just that problem.but the prophecy was that one cannot live fully with the other still alive or summin like dat.mayb the prophecy meant that the heir of gryffindor cannot live while slytherin's lives and that full power is only achieved if the generation is wiped out. mayb the line of gryffindors will only achieve full power by killin slytherin's.cause it was godric and salazaar who started beefin first and so mayb they cursed eachother in this way?