You should heat up the first piece, form it to shape and cool it off immediately, and then heat up the second piece and make it conform to the shape of the first piece. Trying to shape two pieces at once would mean that you'd have one sliding against the other -- which is possible, but I wouldn't trust it to work all that well.
here at work when we get bubbles in veneer we use a heat gun to release the glue bond from the wood. I imagine the same applies her especially when bending material like elk said you get sliding from your top layer. Look at how pringles stack, same size chip but the edges dont overlap when ontop of one another.