Re: AC filters
I have used a double filter system, and it worked quite well. I do want to emphasize what the others have said about restriction of air flow, it is a bad thing.
In my situation, I had a furnace with an intake at floor level in a high traffic and high dust/debris area. In this location I installed one of those cheapy, blue "wooly" type filters that have zero restriction and are only good for catching rocks, gravel, and small children from entering the duct. In the furnace itself, I used pleated filters to catch all the dust and smalls that the primary missed. I had good luck with this, with no detriment to the system.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!