How to limit the post processing effect only to the base camera

Hi everyone! I'm making a 2D game with perspective cameras and was planning to create different visual effects on different objects.

I've created two different cameras culling different layers, one stacked on the other as an overlay camera, and found that the post-processing effect for the base camera automatically applied to the stacked one. Since I only want the base camera to have the effect, I've unticked the 'post-processing' option for the overlay camera and changed its volume mask to nothing (not sure if this helps, went for random tries....), while this didn't work, I also the made the two cameras in different positions after adding a small local volume covering only the local camera, but this didn't work either - the camera shares the effect as long as it is stacked to the base one.

Had experiences using the same approach with an orthographic camera in an older version of Unity before and it worked all fine. Not sure if I did anything wrong this time or I got the whole idea of camera stacking/post-processing wrong.

Thanks in advance!!