Can multiple pieces of crown molding be stacked / combined?
Yes, if you apply a plywood backer that's about 3/4 the width of the assembly. Crown mold takes a lot of stress and strain during installation, from fitment, mitering, coping, and nailing. Those thin joints are likely to crack apart during the process. 

I'd back it up with either 3/8" BC plywood, or 1/4" birch underlayment (Tecply).