The Gemara in Masechet Pesachim (109-110) discusses the potential danger involved when one performs activities involving pairs of two, such as drinking two cups of a given beverage, or eating two of the same food item. Doing things in pairs posed risks involving certain mystical forces and demons. Must one be concerned about this danger nowadays?
The Tur (Orach Chayim 170) indicates that the concept of "Zugot" ("pairs") indeed applies even nowadays. The Beit Yosef (commentary to the Tur by Rabbi Yosef Karo, author of the Shulchan Aruch) questions why the Tur mentions this concept but omits the details. The Gemara discusses many conditions upon which the potential danger of "Zugot" depends, and the Tur makes no mention of these conditions. In any event, the Beit Yosef himself was of the opinion that we are no longer under the influences of these spiritual forces, and thus the notion of "Zugot" does not apply nowadays. This is also the position of the Chid"a (Rabbi Chayim Yosef David Azulai, 1724-1806). Likewise, the Ben Ish Chai (Rabbi Yosef Chayim of Baghdad, 1835-1909), in his work of responsa "Torah Lishma," writes that these forces have diminished in strength over the years, and therefore one need not be concerned about the dangers of "Zugot." In fact, he records a practice in Baghdad that when a person celebrated a happy occasion, the women in the community would specifically send the person two baskets of gifts, in order to demonstrate that there is no reason to be concerned for the effects of "Zugot."
Nevertheless, although the dangers of "Zugot" do not apply nowadays, there is one practice that many people still observe regarding this concept. Namely, people who must visit a mourner's home a second time make a point to pay a third visit, as well, so as to avoid making a pair of visits. Chacham Baruch Refael Ben Chayim Z"L mentioned that although strictly speaking one need not observe this practice, he nevertheless ensures to avoid making a pair of visits to a mourner's home because he saw many great rabbis follow this practice.
Summary: Nowadays there is no need to be concerned for the potential dangers of "Zugot," performing activities involving pairs. Nevertheless, many people have the practice to pay a third visit to a mourner's home if they had to pay a second visit, so that they will not have made a pair of visits.
