If you can fit both birds on the same shelf with the shelf as high in the smoker that you can and still have room for you meat probes, that would be the best option. If you are able to do this, the extra bird will not add too much to your time. Maybe 30 minutes give/take.
If you can't fit them on the same shelf, you will want to rotate them (top to bottom and front to back) after 2.5 hours or so. Otherwise one will get done way before the other.
Happy Thanksgiving! I hope you birds turn out well.