This video shows the process of making a two piece moon jar. It starts with making two large bowls of exactly equal diameters. Obviously, while throwing the second one you've got to work it so that it matches the diameter of the first. By making a groove in the lip of one bowl and a pointed lip on the second, the two bowls can be joined together perfectly (so long as their diameters match!). I learned this tip from Dave Kim the Potter at Sculpture Space who made this video for me.