I have my home theater for the most part worked out accoustically except for the ceiling and the floor...

It is a bonus room on my 2nd story that will be converted and there is carpet and carpet pad already in the room. I was wondering if just screwing a second layer of plywood on top of the carpet pad that is already there would give me the same, or close to, the effect of putting a second layer of plywood with green glue in between. We would then put ANOTHER layer of carpet pad on top of that and then the carpet. My son's bedroom and our guest bedroom are under this room and I am trying to isolate it as best we can. If this is not a good idea (maybe due to the thickness of the carpet pad)...would we be able to use that thin laminate floor underlayment between the 2 layers? Or are we just really better off using the green glue.

Any thoughts/opinions/experience would be appreciated.

Screwing a second layer of plywood on top of the pad will achieve nothing. Plus the screws you will using to hold down the second sheet of plywood will create a direct path for sound to travel through. You efforts in sound isolation will be in vain. If I could suggest a product called AcoustiK Mat GLUED to the original subfloor (you would have to tear up the original pad and carpet) then glue 1/2" tongue and grove plywood on top of it. The only trouble with this method is that it will probably raise your floor up about an inch which would mean trim would have to be removed and reinstalled and you will most likely have door clearance problems.