I prefer using a single plate for the soundboard and even the back but larger instruments, like the baritone uke and the terz guitar, require joining two bookmatched plates. To join two plates that are 1/8 in. thick edge on edge requires some doing, especially with fairly rudimentary tools. The difficult part is to get the edges perfectly matched to each other before gluing so that you cannot see any light in-between the plates. It's easier for soundboards because the plates are softwood and tiny gaps will disappear when the plates are pressed together in the joining process. Hardwood plates for the back are more exacting. I also like to insert a strip of contrasting wood between the two back plates, which makes it a double joint: first gluing the strip to one plate and then gluing that assembly to the remaining plate.
Here are a few pics of a spruce soundboard held against the sky to check that the seam is indeed flawless, and of backs (maple, walnut and lacewood).