The forward and rearward balsa sheets will ultimately get glued together. The strongest way to glue the sheets together is with a scarf joint. The pic below shows the basic approach (viewed from the side edge). A scarf joint 6 times the thickness of the material gives a good strong joint. The easiest and quickest way to accomplish this with 1/8" material is to lay the pieces on top of one another, place at the edge of a hard flat surface (edge of a workbench or a piece of mdf), slide the upper piece over 3/4", then remove material with a rigid sanding block with coarse paper. When the upper piece is rotated over, it will fit the lower piece very tightly. This is a quick and easy process, but a little practice on scrap wood is worthwhile before attacking a large balsa sheet.

I decided to glue the pieces together after molding as I was concerned that a glued up scarf joint might interfere with the molding process.