spawning random objects at 2 defined points

hey all, so ive got two empty’s set up , one on the top and one on the bottom of the screen.
I have 4 different shaped objects that are prefabs, what i wish to do is take it in turns every second or so and spawn 1 of the 4 objects either at the top empty of the bottom one, taking it in turns with each spawn location. i have no idea where to begin lol. i have got an outline of what needs to happen

-1 second PASSED
-random pick object out of 4 prefabs
-spawn object at top -CHANGE ACTIVE SPAWN TO BOTTOM-
-1 second PASSED
-random pick object out of 4 prefabs
-spawn object at bottom -CHANGE ACTIVE SPAWN TO TOP-


If you put this in a c# script, define the top and bottom position in the scripts inspector, and the prefabs in the array in the inspector, this will keep spawning a random object randomly, switching from a top position to bottom position

    public Transform[] objects;
	public Vector3 topPosition, downPosition;
	private bool spawning = false, isTop = true;
	void Update()
		//if not spawning, start a new spawn
	IEnumerator Spawn()
		spawning = true;
		//select random object out of array
		Transform randomObj = objects[Random.Range(0, objects.Length)];

        Transform clone;
			case true:
                clone = Instantiate(randomObj, topPosition, Quaternion.identity) as Transform;
                isTop = false;
            case false:
                clone = Instantiate(randomObj, downPosition, Quaternion.identity) as Transform;
                isTop = true;
		yield return new WaitForSeconds(1);
		spawning = false;