Is the liquid I get after pressure cooking a whole chicken a "complete" stock? Recently I've been buying whole chickens (~4-4.5 pounds) from the grocery store and pressure cooking them to great effect. It cooks fairly well, and I add about 1.5 cups of water to get it started. When the chicken comes out, I end up with just under a quart (4 cups) of liquid and fat. I strained off the fat and ended up with a liquid that got beautifully gelatinous overnight in the fridge.

In previous attempts, I've always taken this liquid along with the bones and little bits of meat, added more liquid to get to about 8 cups total, added aromatics, and pressure cooked it again. I end up with much more liquid (obviously), but it doesn't end up as beautifully gelatinous as the initial liquid.

My question is this: if my initial cook is in the pressure cooker, should I expect any additional extraction of flavor or gelatin from the carcass and remaining bits, or is it likely that all the substantive extraction took place on the cook of the whole chicken, and that the liquid I got the first go-round is all I'll get?
If you are cooking the chickens whole, and I think you are from your description, then your cooking liquid has not had sufficient contact with the bones to extract the available gelatin. You should absolutely be able to make a stock with the carcass later.

As you say, even with the nice gelatinous cooking liquid you have from your first step, you don't have a whole stock without the aromatics. My advice is to make your stock with the carcass as usual, but add your cooking liquid too for a nice boost.