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I am assuming your home is ducted and has a forced air system. If you you only have 1 furnace and 1 condenser, it my be more energy effecient to expend to two units. I recommend Lennox. You can also look at installing spilt units like these made by Fujitsu:
These are designed so that you can cool a room/rooms individually. I suggest calling a few licensed HVAC contractors and get 3 quotes.
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