How to async load new Scene and Resources simultaneously?

Hey, I need some help on how to properly load resources at scene transitioning without noticing any lag. I searched the web for this, but it seems there are only answers of asyncronous loading just the desired scene.

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My game is constructed by having a menu scene (=current scene) and a game scene (=target scene).

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From the menu I can select a particular level. Each level is a prefab, which should get loaded into the game scene.

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The prefab-loading should be part of the scene transitioning procedure as loading afterwards makes no sense. The application should load the prefab after the target scene is loaded (async.progress >= 0.9f) and after both are loaded the scene should activated.

This is what I have, but it will loop forever because allowSceneActivation = true is never reached.

 private IEnumerator LoadScene(string level, string prefab = ""){
     float loadcount = prefab == "" ? 1f : 2f;

     AsyncOperation async = SceneManager.LoadSceneAsync(level);
     async.allowSceneActivation = false;

     ResourceRequest prefabRequest = Resources.LoadAsync(prefab );
     DontDestroyOnLoad (Instantiate (prefabRequest.asset as GameObject));

     while (async.progress < 0.9f) {
         loadProgress = (async.progress) / loadcount * 0.9f;
         progressBar.value = loadProgress;
         yield return null;
     }
     if(prefabRequest != null){
         while (!prefabRequest.isDone) {
             loadProgress = (async.progress + prefabRequest.progress) / loadcount * 0.9f;
             progressBar.value = loadProgress;
             yield return null;
         }
      }
      async.allowSceneActivation = true;
 }