Does a double supercharger really gain you anything? Basically a supercharger forces air into the cylinders (ie, forced induction) rather than having the vacuum created by the pistons draw in the air/fuel mixture. I can't imagine what having a supercharger forcing air into another supercharger would accomplish. In most cases, a single supercharger doesn't provide all the boost it is capable of, otherwise the heads would probably get lifted off of the block.
But heck, I'm willing to learn something new..............