A rococo crest like the one on this showcase is not symmetrical, and carving it is nothing like carving a mirrored pair.
A symmetrical crest can be worked one side at a time. Finish the left, take a template off it, reverse it, cut the right. The work is exacting but the method is simple.
This crest is designed the older way, with the two halves differing - a scroll that runs further on one side, a shell set slightly off centre, an opening that appears on one half and not the other. It is balanced by mass, not by mirroring, and mass can only be judged with both halves in front of you at the same stage.
So our carver works alternately. A little is taken off the left, then the equivalent off the right, then both are stepped back from and weighed against each other before the next cut. If one side gets ahead, the judgement goes with it and the crest ends up leaning.
It is slower than working half and copying it. It is the only way the finished crest sits level over the cabinet without being identical on both sides.