Can I bake 1 round of puff pastry on top of another Can I put 2 layers of puff pastry ( 1 on top of the other ) on top of a dish to make it very high ?
Yes, but you might want to use an eggwash or water to make sure the two layers stick together.  

You'll want to be careful about not brushing near the edge -- if it drips down, it'll glue the side together preventing it from puffing properly