Yes, don't forget to set the new camera Layer to your post-processing layer name.

I tried exactaly this, but still not working. My steps were:

1- Create a normal camera and add it to the AR Camera.

2- Set the second camera Clear Flags to Depth Only (if I don't do this, the background will be a color).

3- Set this new camera Depth to 2, so it apear in front of the AR Camera.

4- Add all my objects to a NewLayer and set this new camera Culling Mask to show only NewLayer.

5- Added Post Processing Behaviour component and added my post processing.

When I hit play the whole image is affected by the effects. 

Is there something I'm missing here?

You could try to render all the objects you want to be affected by the post processing stack in a separate camera. This camera can be a child of the AR Camera. Then you could apply the post processing effects only to that camera.