The Chachamim teach, “A person should always enter through two doorways in the synagogue… and then pray” (Berachot 8a). There are three interpretations of this statement and all were accepted as halachah (Shulchan Aruch 90:20).
The first explanation of Chazal’s words is that one must enter inside the synagogue at least a distance equal to the width of two small doorways (approximately 64 cm or 25.2 in), since one who prays next to the entrance makes it seem that prayer is a burden to him and that he is standing there in order to leave immediately (Rashi). However, if a person’s permanent seat is near the entrance, he is permitted to pray there, for everyone knows that that is his spot and he is not standing there in order to exit quickly (Talmidei Rabbeinu Yonah).
Based on this, it is clear that l’chatchilah one should not pray in the entrance hall of the synagogue, for if Chazal say not to pray inside the synagogue near the entrance, all the more so, one should not pray in the hallway adjacent to it.
The second explanation is that one should not sit close to the entrance, so as not to look outside and be distracted from ones prayers (Maharam of Rotenberg). Accordingly, it is also not proper to sit next to a window facing outside.
The third explanation is that the person coming to pray must pause a few seconds, equivalent to the amount of time it takes to enter two doorways, before beginning to pray, in order to devote his thoughts to prayer (brought by the Rosh).
Additionally, Chazal’s words here allude to two spiritual doorways through which a person must pass before he starts praying. In the first doorway, he must rid his mind of worldly matters troubling him, and in the second doorway, he must direct his kavanah to serving Hashem (see Maharal Netiv HaAvodah, chapter 5).