Algodoo

Insert a magenta-colored ball into one of the katapults. The attached laser recognizes that a ball is inside. If its left, the state A is set to true, if its right, the state B.
Then the states A and B are processed by the logical tool XOR, giving the result P3.
In the orange-colored "Solidmaker" P3 makes S1 solid, if P3 is true, otherwise non-interacting (with collisiongroup A).
The S1-type squares are attached to rotating wheels, which push the light-blue blocks against the springs of the catapults. So, if the S1 is solid, the catapults will receive energy and can shoot the ball.

Efficiency: If there is no ball inside, no shooting happens, which prevents unnecessesary usage of energy driving the springs.

Now add the second magenta-colored ball into the empty katapult . Experiment with it, taking it out again, observe, put it into it again.

The nice thing about this construction is, that a collision of two balls is prevented by the XOR, because if both states A and B are true, then A XOR B will produce P3=false, and S1 will be non-interacting ("transluctent" ) and the catapults will not act.