I don't see it being a major problem. If you set up the tanks so the warm end is near the fireplace, leaving the dragons a cooler area of the cage to retreat too, there shouldn't be any problems with overheating. The only way to know for sure is to place the cage where you are considering, have a fire and monitor how the temps change inside the cage.
For the most part, unless you are making a huge fire (ie using it to heat the room) the temps shouldn't increase too much in the cage, especially if its 6 feet or more away from the side of the fireplace.
Smoke shouldn't be a concern, unless you have a blocked chimney or leaking chimney, in which case the smoke can come billowing out of the fire place or carbon mon-oyxide. This can be prevented by simply making sure the chimney is clear and the chute (flew? whatever that thing is called in the chimney you can open or close to adjust air flow, the name is eluding me right now) is open. To find out about CO, a CO detector in the same area as your dragon will alert you if there is a problem.
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PHLdyPayne