Rough implementation of fully-elastic collision. Only modifies the position and velocity of sprite 1; the assumption is that this code will get run again with the sprites swapped round. The stuff in actionQueue.enqueue should get run after all collisions

elastic-collision.as

var sprite2ToSprite1:FlxPoint = VectorMath.subtract(sprite1.centre, sprite2.centre);
var sprite2ToSprite1UnitVector:FlxPoint = VectorMath.normalize(sprite2ToSprite1);
var sprite2SpeedTowardsSprite1:Number = VectorMath.dotProduct(sprite2.velocity, sprite2ToSprite1UnitVector);
var sprite1ToSprite2UnitVector:FlxPoint = VectorMath.negate(sprite2ToSprite1UnitVector);
var sprite1SpeedTowardsSprite2:Number = VectorMath.dotProduct(sprite1.velocity, sprite1ToSprite2UnitVector);
var sumOfThoseSpeeds:Number = sprite1SpeedTowardsSprite2 + sprite2SpeedTowardsSprite1;
			
var sprite1VelocityChange:FlxPoint =
	VectorMath.multiply(sprite2ToSprite1UnitVector, sumOfThoseSpeeds/2);
			
var distance:Number = VectorMath.magnitude(sprite2ToSprite1);
var distanceAdjustment:Number = ((sprite1.width + sprite2.width)/2 - distance)/2 + 1;
if (distanceAdjustment < 0) {
	distanceAdjustment = 0;
}
var adjustmentVector:FlxPoint = VectorMath.multiply(
		sprite2ToSprite1UnitVector, distanceAdjustment);

if (sumOfThoseSpeeds > 0) {
	actionQueue.enqueue(function () :void {
		sprite1.velocity = VectorMath.add(sprite1.velocity, sprite1VelocityChange);
		sprite1.x += adjustmentVector.x;
		sprite1.y += adjustmentVector.y;
	});
}