Fireplaces, Heating Stoves, Flues and Chimneys - 2 fireplaces; smoke in house

We just bought a house constructed in the 50s. There are 2 fireplaces, one on the main floor and one in the basement. Each has its own chimney, but they are close together (about a foot apart on the roof). The 2 fireplaces do not appear to be connected at all. A chimneysweep cleaned them this fall and said that everything looks good.

BUT...when we light a fire on the main floor the basement fills up with smoke. Since I can't see a connection between the 2 fireplaces I suspect that the smoke going out of one is somehow being sucked down the other (even though the flue for the basement one is closed, it's not airtight).
Do you think the problem is what I described and how would one go about fixing this. Or am I on the wrong track; would something else be causing this problem?

I have the exact same issue! 2 stories with 2 fireplaces with chimney outlets about 1-2 ft apart. We live on the second story and use the fireplace every so often and had the basement filling up with smoke. We eventaully ended up putting a solid cover over the chimeny outlet for the basement and that "fixed" the problem. Not really a solution if you want to use both fireplaces, but a possibility :)

Negative pressure in the basement. Open a basement window and it will go away. :D Install top mounted dampers to seal off the unused flue when not burning. A combination rain cap/ animal guard/ damper is best.

Get a level II inspection. go to www.f-i-r-e-service.com for a certified fireplace inspector near you.

Hth,
hearthman