Nested Post Process Volume that influences part of camera frustrum

Trying to enable nested Post Process Volumes in my scene but so far not working the way I want

What I’ve done.

1 Create a ‘global’ PPV with extents set to infinite and Priority set to zero.
2 Created a 2nd PPV and sized to just the area I want to be different. Priority set to one and Infinite extents disabled.

It seems that PPVs are only designed to influence the cameras they contain.
Is there some way to have a PPV influence just part of a camera frustrum, ie a specific region?