Random Selection of Objects for reaction time experiment?

Hi there,
I have a simple scene containing an avatar, a table and 4 different objects on the table.
I would like to randomly select pairs of two objects display them for 5 seconds and then randomly select a new pair.Every pair should only be displayed twice. This should proceed for a certain number of iterations.
Code I have so far: I have a class that deactivates all the objects at the beginning, calls a function that generates a random number and selects another function that activates and deactivates the pairs and checks how often this function has been called.

My Problem: It works for one iteration and then it does nothing. Could it be that the problem is, that I am using "yield WaitForSeconds(time)" in the "showBottleCup" function togehter with a for-loop?
I need something like: Wait for the function to activate/deactivate the objects and then go on with the next iteration.
What could the problem be? Is there a better solution to it?

I am thankful for every idea!
Thanks a lot!
Adrian

public class TrialController : MonoBehaviour {
    public GameObject bottle;
    public GameObject cup;
    public GameObject lamp;
    public GameObject book;
    private int NumBottleCupShown=0;
    int numberOfLoops=3;
    int timeIntervall=5;
    int randomNumber;
    void Start ()
    {
        cup.SetActive (false);
        bottle.SetActive (false);
        lamp.SetActive (false);
        book.SetActive (false);
        showRandomPair(this.numberOfLoops, this.timeIntervall);
    }
    void showRandomPair(int numLoops, int time)
    {
        //Loops for the number of permutations needed
        for(int i=0; i<numLoops;i++)
        {
       
            //Generate random Number in the intervall of 0 to 3
        this.randomNumber = Random.Range(0,3);
            switch(randomNumber)
            {
                case 0: //StartCoroutine(showBottleBook(time));
                break;
               
                case 1: StartCoroutine(showBottleCup(time));
                break;
                case 2: //StartCoroutine(showBottleLamp(time));
                break;
               
                default: Debug.Log ("Something went wrong");
                break;
            }   
       
        }
    }
    IEnumerator showBottleCup(int time)
    {
                   if (this.NumBottleCupShown <= 2) {
       
                        NumBottleCupShown++;
                        cup.SetActive (true);
                        bottle.SetActive (true);
                       
                        yield return new WaitForSeconds(time);
                        cup.SetActive(false);
                        bottle.SetActive (false);
              
                }
        }
       
    }