SceneCombiner.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.SceneManagement;

// author: mvaganov@hotmail.com
// license: Copyfree, public domain. This is free code! Great artists, steal this code!
// latest version at: https://pastebin.com/raw/8cZ5yKSd
// works well with Singleton: https://pastebin.com/raw/JMme8A2X
public class SceneCombiner : MonoBehaviour {
    [Tooltip("What scenes to combine together.\nConsider having objects with 'Singleton' in the Scenes to prevent duplication.")]
    public UnityEditor.SceneAsset[] scenesToAdd;
    // Use this for initialization
    void Start () {
        for (int i = 0; i < scenesToAdd.Length; ++i) {
            SceneManager.LoadScene (scenesToAdd [i].name, LoadSceneMode.Additive);
        }
    }
}