You really cannot layer carnauba waxes. The oils and solvents in them sort of reactivate the previous coat so all you end up doing is smearing around the product. An extra coat basically just evens out the protection and shine, it doesn't increase them for the most part.
Impossible to say accurately how long a wax will last as there are so many variables that affect durability. In general most good carnaubas like S100 will give you 4-8 weeks of protection, maybe more if the car is garaged and well cared for.