We know that momentum is conserved in all 2D collisions. The main difference between 2D collisions and 1D is that in 2D collisions there are two components of momentum. If one ball is moving to the right and hits another ball at an angle, then the rightward momentum is conserved. However, there is an upward/downward component. If you know the angle of separation between the two balls, then you can use trigonometry to solve for the momentum of each ball! Here's a problem with an explanation on how to solve a momentum problem by using vectors!
