EJSS 2 Cannon aim at each other Projectile Motion Model m=1kg, g=-9.81m/s^2

is conceptually in the green line being without the effects of gravity, the cannons ball will meet as predicted by Newton's 1st Law of motion without net external force.
now, adding the same gravitational acceleration means both cannon balls will move down at the same rate ( pink lines).
therefore, the 2 cannon balls has to collide .