I have the same smoke problem at my house. When I get the downstairs fireplace going, smoke enters the upstairs fireplace. Upon inspection I found the although both fireplaces have good top dampers, the smoke is migrating from one flue pipe to another. Poor masonary work is to blame. The mason got away with not sealing flue pipe sections when one fireplace was installed but adding the second fireplace flue inches away introduces smoke when negative pressures are present within the house. I think I may have to install a stainless steel liner with a top damper. Hope it works.