EditorGUI scrollview within a scrollview... is it possible?

I just created 3 nested ScrollViews and a button inside and it works as expected. Since you have problems with your code it might help when you share the code that doesn't work for you. Just edit your question.

That's my test:

 Vector2 scrollPos1;
 Vector2 scrollPos2;
 Vector2 scrollPos3;
 void OnGUI()
 {
     scrollPos1 = GUI.BeginScrollView(new Rect(10, 10, 300, 300), scrollPos1, new Rect(0, 0, 500, 500));
     scrollPos2 = GUI.BeginScrollView(new Rect(10, 10, 200, 200), scrollPos2, new Rect(0, 0, 500, 500));
     scrollPos3 = GUI.BeginScrollView(new Rect(10, 10, 100, 100), scrollPos3, new Rect(0, 0, 500, 500));

     GUI.Button(new Rect(0, 0, 500, 20), "Test");

     GUI.EndScrollView();
     GUI.EndScrollView();
     GUI.EndScrollView();
 }

That's how it looks like:

Maybe you expected a different behaviour but without more details we can't say anything what might go wrong on your side.

edit to explain a bit more in detail what happens in my example. I create 3 nested scrollViews and each has a virtual size of 500x500. The outer scrollview has a size of 300x300. The second scrollview is placed inside the virtual space of the first. So i can scroll the outer scrollview to the right and the nested view will disappear to the left (since we only see a 300x300 window of the 500x500 area). For the innermost scrollview it's the same