I love paper mache, and I do a lot of this with my private art
students. First of all, it pays to buy strong balloons. I know
that's not too helpful now, but a good thing to remember for
next time. Instead of buying the cheaper balloons in a
package, buy them singly at a party store.
When I have work deflate, I carefully remove the first
balloon, insert a new one, and blow it up to the necessary
size and tie a knot. Continue adding layers, and no, two
layers will not be strong enough. As a matter of fact, if you
stop with two layers, the shape will probably deflate when you
remove the balloon.