I agree with Bob, you need to provide a combustion air source. You're experiancing stack effect and it can be overcome. Below are a couple of links that will help explain the phenomenum. You can also provide an air source in your basement by drilling a hole through your sill plate and installing a clothers dryer vent with ducting to a register grill mounted in the ceiling. Just open it when burning in the basement or instead of that you can open a basement window when you burn.
http://www.thechimneysweep.ca/2stackeffect.html
Possibly you're also experiancing some crossover since your flues are so close together.
http://www.masonrymagazine.com/6-03/fireplace1sb.html