Spawn help, 4 spawners but need one object on screen at a time

I have a 2D game that I am building and I need help with my spawners. I have a spawner to the left, right, top, and bottom of the camera that spawn objects that go to an object in the middle of the screen that rotates to catch them. I have the spawners set up to spawn the object at a random time but objects often reach the middle at the same time making it impossible to catch them both. I have this one code on all 4 spawners. I need a way to merge the spawning of all 4 or a box that checks if an object is already on screen before it spawns one or not. All my code is in C# and this is my first project.

using UnityEngine; using System.Collections;

public class RandomTop : MonoBehaviour {

 bool isSpawning = false;
 public float minTime;
 public float maxTime;
 public GameObject[] prefabs; 
 
 IEnumerator SpawnObject(int index, float seconds)
 {
     
     yield return new WaitForSeconds(seconds);
     Instantiate(prefabs[index], transform.position, transform.rotation);
     

     isSpawning = false;
 }
 
 void Update () 
 {

     if(! isSpawning)
     {
         isSpawning = true;
         int enemyIndex = Random.Range(0, prefabs.Length);
         StartCoroutine(SpawnObject(enemyIndex, Random.Range(minTime, maxTime)));
     }
 }

}