This thread is highly relevant for you:
They used camera stacking to get both 3D and 2D elements, BUT in more recent versions of Unity it is broken. So it appears the answer is that you must use an older version of Unity.
This thread is highly relevant for you:
They used camera stacking to get both 3D and 2D elements, BUT in more recent versions of Unity it is broken. So it appears the answer is that you must use an older version of Unity.