What I do is dop both stones on different dops. I then cut the first stone to the size and basic shape that I want and them I take take the dop out and put the second stone in the quill and cut it to the exact dimensions, without making any changes to the depth of the cut. I even go as far as cutting all of the crown facets the same way, setting up one tier, cutting it and then changing out the dops and cutting the same facets on the other tier before going onto the next tier.