can I use same pot of pickle for brass and copper
Sure. But don’t leave the brass in there too long, because the pickle will leach the zinc out and leave it porous.
Yes, but standard pickle will leave a copper finish on your brass. So to get the brass color back you can refinish it past the copper coating, or just make up some super pickle with standard pickle granules dissolved in hot water and add some hydrogen peroxide to it. It’s only good for a few hours.