I would empirically try it out. Just because there is a steel door does not mean the wall is also solid steel, or that the door edge does not leak RF energy.
Just make 2 antennas as one loop , where each is half of te original - look like serially connected coils. Impact will be i reduced senstivity equ decrease of distance. That is one solution .
Second is to use analog HF switch (relay and others) to select antenna to use and sequentially switch between antennas - means longer response to read transponder .