I never knew this. Thanks Tommy.Joining two shells together is how all the steam benders make any drum deeper than 8-9 inches. That's why Craviotto likes to use an inlay on their drums. It hides the seam.
As for why you would do it on a ply shell instead of just buying a shell in the proper depth is a bit if a head scratcher to me, but to each their own.
I mean, it may be fun to have one side be maple and the other side be walnut so you can flip the drum and possibly produce slightly different sounds, but joining two of the same species together is not as fun sounding.