Venting

Air can still be pulled through the furnace into the chimney weakening the coal stove's draft - possibly to the point of getting a draft reversal due to stack affect of the house sucking it back through the coal stove and releasing carbon monoxide into the house. Don't forget that your house can act like a chimney, too. Enough of one that it can over power the weak draft of a chimney. Then as Mom used to say, You'll wake up dead."

A duct baffle won't seal well enough and depending where the baro is in the stove pipe, and how it's adjusted for the furnace, it may not work with the stove. You might have to disable the baro when the coal stove is operating?

Sounds like what you would need are duct "slide gates" in the stove pipes to completely close off air intrusions to the stove pipe/chimney system coming from each appliance.

Then, to be safe, turn off the power to the furnace, close the furnace slide gate, and then open the slide gate for the stove whenever you start the stove.

Also understand that very likely, doing that would still not be allowable with any codes.

Paul

Thanks for the input guys I'll look into the slide gates because by the sound of it that's what I was looking for so that's the best device to close off air flow completely in stove pipe?