The Prussian 1812 drill regulation for artillery prescribed for fortress and siege artillery the following positions (numbers):
light cannon and howitzer: 5 men
heavy cannon: 7 men
heavy howitzer: 8 men
light mortar: 3 men
heavy mortar: 5 men
plus 1 NCO for every two or three guns.
Of course, during the loading process one single man could fulfill the tasks of several positions one after the other, and after that even aim the gun. I think this is the clue for the low number of 2 artillerymen required.
For lifting the bombs of the heavy mortar, two men are deemed to be sufficient. Fetching the cannon balls, grenades and bombs, and putting them into the barrel, is easy enough so that it could well be done by infantrymen after a short instruction.
Moving the guns forward and back, is much easier than in the field artillery, as the batteries are provided with a wooden platform: no mud, no inclination of the ground.