what point of time the bifurcation of Vadakalai and Thenkalai sects took place and for what reason?

“Dear sir,

At what point of time the bifurcation of Vadakalai and Thenkalai sects took place and for what reason? What are the differences in sampradayam? Is it necessary (for vaishnavam to strengthen) to attempt a unification of these sects?”