Large ukuleles, tenors and baritones, generally use two pieces of book matched backs. Here is how I do it.
The most important step is joining the edges perfectly. First I plane each piece, then I place a long strip of sandpaper on my table saw surface and sand until I can see no light between the two edges. Take your time and seek perfection, this is important.
Once the edges mate perfectly, lay down a strip of visqueen both below and on top of your joining edge. then apply the glue, being sure both surfaces have received glue.
Here is a simple, but effective way of clamping the two pieces together, that is if you have a 25 lb bag bird shot.
The pieces and parts for #s 116 and 117 are slowly being assembled on the work table. A tenor and a baritone in their early stages. Tomorrow I will be thinning the backs and perhaps bending the sides. Stay tuned.