Render two cameras separately (I'm using a custom Blit render pass)

SO I was actually looking around, and it seems like this is an existing issue, and a lot of people are asking for the feature HERE !
It also seems like using render textures is NOT a possibility as URP doesn’t preserve alpha channels post render, and there doesn’t seem to be an easy way to access the OVERLAY cameras RTs (but honestly I just haven’t looked into it).
However I am not sure if the setup I specifically have could help in your case:


this is the general setup that I have:
The whole scene is in the DEFAULT LAYER, except for the “RED RING” and also in this case the “GLOBAL VOLUME” that I am using as a fancy POST EFFECT to affect only the RED RING. In your case, you probably wouldn’t need this, as you’re doing your own camera blitting.
One thing that is important though, is the render order of the overlay cameras.

As we spoke before, each of the child Cameras are set to OVERLAY, but the one which is rendering your DEFAULT LAYER, should not CLEAR DEPTH. (In this case I also have it ignore all volumes, which would also not matter if you have a custom effect)
BASE CAMERA:

and here is the outer RED RING CAMERA, which is using the VOLUME LAYER to apply the effect.

If it’s all set up properly, this SHOULD work. HOWEVER, this is a pretty limited setup and it will only mask the OPAQUE pixels of the RING CAMERA (for example, It will likely not work if you want to make a specific object be blurry or have a glow), and due to the URP RENDER TEXTURE issue, I am not sure if you can actually use the alpha channel to have a nicer effect either.

In any case, I’m no expert myself but that’s all I could come up with. Hope it helps.