A popular question, but it does not have a answer.
How to do it? People told about several cameras and culling layers. Yes, it’s logical…
.
But blur post effect affect all layer below yourself, so I cannot place blur effect on the top layer. Only bottom layer can be blured. This is not what we want.
So, this is impossible? 
Also people told about shader replacement DoF system. I go to example Unity - Manual: Replacing shaders at runtime but… In the example project only 5 scenes, not 7. And DoF example is missing.
Mmm… I remember that I seen DoF example, but where is it? )