Multiple SceneManagers in one Scene

CoexistiveSceneManager.cs

using System.Collections.Generic;
using UnityEngine;

//All SceneManagers that I want to keep in one scene needs te extend this class.
//I had to fix it in designer generated code which is poor, or it can be copy pasted to each Scene class.
//Add all root GameObjects for each scene so they will get enabled when scene is activated
public class CoexistiveSceneManager : SceneManager
{
    public List<GameObject> SceneRoots;

    public override void Setup()
    {
        base.Setup();
        SetSceneRoots(false);
    }

    public override void OnLoaded()
    {
        base.OnLoaded();
        SetSceneRoots(true);
    }

    public override void Unload()
    {
        base.Unload();
        SetSceneRoots(false);
    }

    private void SetSceneRoots(bool value)
    {
        foreach (var sceneRoot in SceneRoots)
        {
            sceneRoot.SetActive(value);
        }
    }
}

LevelSelectScene.cs

public class LevelSelectScene : LevelSelectSceneBase {
    public override System.Collections.IEnumerator Load(UpdateProgressDelegate progress) {
        // Use the controllers to create the game.
        yield break;
    }

    public override void Setup() {
        base.Setup();
    }

// Replaced autogenerated commands from SwitchGameAndLevel to SwitchGame.
    public override void Back()
    {
        GameManager.SwitchGame<PackSelectScene>((container) => { container._PackSelectSceneSettings = _BackTransition; });
    }
}

LinkToDesign

https://www.dropbox.com/s/ngwojkkbyde6k47/SceneManager%20design.jpg