The correct equation that represents conservation of momentum for two colliding objects is:
m1v1 + m2v2 = 0
This equation indicates that the total momentum before the collision (m1v1 + m2v2) is equal to the total momentum after the collision, assuming the system is isolated and no external forces are acting on it. If we were considering the change in momentum, we would typically set the initial and final momenta equal to each other, but in this case, we can express that as the summation being zero for objects in opposite directions.