[Tunafish]

So post (after/behind) will be exhaust leaving cat, cat to muffler.. Pre (before/front) will be exhaust entering cat, cat to head.
You have two sensors behind the cat, one on the exhaust manifold after one of the cats and other is on the connecting pipe that you pictured.. Which one is 1 and which one is two I am not sure but would guess two would be the one on the connecting pipe.. Part 11787569968