Is there a reward for two people gathering once or twice a week after the Asr prayer to study a religious book, and what are the hadiths that speak about the virtue of gatherings of remembrance (majalis al-dhikr)?

There is no harm for the questioner in studying a religious book and dedicating time for it weekly; rather, he will be rewarded for it, as it is a pursuit of knowledge and one of the gatherings of remembrance (dhikr) that angels encompass. For more information, one can refer to the fatwas related to the virtue of gatherings of remembrance, their etiquettes, and recommended books.