CameraManager.cs

using UnityEngine;
using System.Collections;

public class CameraManager : MonoBehaviour {
  public Camera cameraOne;
  public Camera cameraTwo;
  // etc.
  
  public void Start() {
    // Switch to camera one
    cameraOne.enabled = true;
    cameraTwo.enabled = false;
    
    // Or switch to camera two
    cameraOne.enabled = false;
    cameraTwo.enabled = true;
  }
}