If you shut down one engine bleed or pack then less air is coming in so less air is let out.
One engine is capable of producing plenty of volume of air to operate the packs one pack is slightly less capable. The MEL altitude restrictions are there more for the purpose of loss of your other source of air and max ability of the apu to operate at altitude. This is a general statement for commercial aircraft in my experience as I am not intimate with the A320. The APU generally produces a lower volume of air than an engine does.