I have done this several different ways. We had a two sided masonry fireplace that was terribly ineffcient. We were able to bring outside air into the combusion zone and install prefabricated brass doors that we sealed onto the brick face of the fireplace. This was not leak-tight, but it at least stopped the loss of heat. We found by playing with the dampers and outside air, we could leave one side open and actually gain heat.
In our current home, we have a large masonary fireplace with an arched opening. We installed a high-efficiency woodstove insert (Avalon) into the opening and used a custom cut shield to fill the opening. The fireplace now produces significant heat and even has ventilation fans to recirculate more heat into the room. The 6" stove pipe is routed through the damper up the chimney, and terminates with a spark-arrestor cap.