It might be quite feasible to add a two sided unit in this space. Since the flue is already there, it makes things even easier. You will need to take out the old unit behind the wall and replace it with a new one that is made to be two sided. You will end up disturbing the finishes on both sides of the wall though. That might be an opportunity to sort out this side of the fireplace with a better approach on either side. Might be nice to create a more symmetrical composition around the new fireplace. May two matching niches on either side...