Trying to Create a grid for a matching game

So I am trying to create a grid for a matching game. I want to take eighteen prefabs and spawn them in a six by six grid so that there are only two of each.

Here is the code that I use to spawn just a regular six by six grid of just plain old 100x100 square sprites:

     GameObject prefab;
     [HideInInspector]
     public GameObject gridHolder;
     public int rows = 6;
     public int cols = 6;
     float spacer = 0.1f;
 
     public static GridManager instance;
 
     void Start()
     {
         instance = this;
         prefab = Resources.Load<GameObject> ("Prefabs/Square");
         defaultMat = prefab.GetComponent<SpriteRenderer> ().sharedMaterial;
         gridHolder = new GameObject();
         gridHolder.name = "Grid_Holder";
         BuildGrid ();
     }
 
     void BuildGrid()
     {
         for (int i = 0; i < rows; i++) {
             for (int j = 0; j < cols; j++) {
                 GameObject square = Instantiate (prefab, gridHolder.transform);
                 Vector2 newPos = new Vector2 (gridHolder.transform.position.x + j + (spacer*j), gridHolder.transform.position.y + i + (spacer*i));
                 square.transform.position = newPos;
                 square.name = "Square_" + i + "_" + j;
                 square.GetComponent<SquareScript> ().row = i;
                 square.GetComponent<SquareScript> ().col = j;
             }
         }
         gridHolder.transform.position = new Vector2 (-2.75f, -2.75f);
     }

My question is, how can I alter this to not only pull from multiple different prefabs but also make sure to use all of them twice, but no more than twice each? I'm looking for the most efficient way to do this.