If you had two breakers from the same phase, you would not be able to get 240volts. You would get a meter reading of 0 between both hots in the connection box of the fireplace. You will only get 120volts. Now if both breakers are using the same neutral (same phase), you can have excessive current on that neutral. Also, if one of the breakers trips, there will still be voltage present from the other circuit in the connection box. I would not borrow or rig anything. I would homerun a 12/3 cable back to the panel as a dedicated line if it needs a 240volt branch circuit.