If I had a large window air-conditioner could I cool my whole house?

I plan on getting central air but right now I about going to get a 15,000 btu air conditioner for my 350 sf downstairs. Upstairs is about about another 350 sf. and has a smaller 8000 btu unit in one bedroom. My question is this: Could I use my forced hot air unit (without the heat) to somehow circulate the air evenly throughout the house? I mean the 15000 is overkill downstairs. I know that central air is more practical, but I am just wondering if somehow hypothetically this is possible.