Michael,

As you undoubtedly noticed, neither Rambam nor Tur make any mention of these "warnings" of Chazal.

Although I didn't see anything about the particular question you asked, we did write about whether Zugos applies today, in our Insights. I am including a copy of what we wrote there, and you can apply it to this Agadah as well as you see fit, since the hazards of a Nidah are also undoubtedly related to Shedim and Kochos ha'Tum'ah. (I'm afraid that at the very least, this method of assassination is not "tried and true" any more.)

Be well, Mordecai

====================

Insights to Pesachim 109b

[II] "ZUGOS" TODAY

What is the Halachah? Do the stringencies of Zugos apply today?

(a) The RASHBAM (110a, DH Rabah) writes that as time went on, the concern for Zugos decreased more and more. We find that even in the times of the Amora'im, the concern for Zugos was decreasing, as the Rashbam explains.

TOSFOS (Yoma 77b and Chulin 107b) explains that nowadays (that is, during the times of Tosfos), we no longer show any concern for Zugos at all. Apparently, he explains, the forces that had power over Zugos have become so diminished that they are extremely rare and do not warrant any concern.

However, the TUR (OC 170) briefly writes that one should not eat or drink anything in pairs. He also mentions (OC 183) that a person should not recite Birkas ha'Mazon after drinking two cups of wine during the meal, because he will be so worried about Sheidim harming him that he will not be able to concentrate on the blessings. The BEIS YOSEF there writes that the Tur should have omitted this Halachah, because Tosfos writes that the Sheidim are no longer prevalent, and in the SHULCHAN ARUCH he indeed leaves them out. The Acharonim suggest various answers as to why the Tur mentions Zugos at all.

Perhaps the TUR mentions the Halachah of Zugos with regard to Birkas ha'Mazon because in the case of Birkas ha'Mazon, it is a matter of one's ability to concentrate on the blessings. A person might not be able to concentrate if he drank two cups during the meal, because he is so worried about being harmed by Sheidim. Even though, in reality, there is no longer any concern for Sheidim, some people are still worried about them, and such a person's mind will not be clear and calm enough to concentrate when he recites Birkas ha'Mazon. Therefore, the Tur says that he should not recite Birkas ha'Mazon after drinking two cups.

Why, though, does the Tur mention that a person should avoid eating or drinking in pairs? It could be that even though right now the Sheidim are no longer prevalent, and in everday practice we do not have to be concerned with eating Zugos, nevertheless, the Tur means to caution that we should never enact a decree or create a custom that involves eating or doing something in pairs. The reason for this is twofold. First of all, perhaps the Sheidim will return. Furthermore, the Gemara (110b) says that even when the Sheidim have no power, "Kishuf," witchcraft, still has power over Zugos. That is, if a sorceror sees someone eating Zugos, he can harm him more easily. Although, we are normally not concerned with the possibility of "Kishuf," since it is rather rare to meet up with a sorceror, nevertheless, a decree or custom that will put a person in danger of Kishuf should be avoided.

QUESTION: We might ask, how could the Rabanan make an enactment to eat Lechem Mishnah -- two loaves -- on Shabbos? That should be a problem of Zugos just like drinking the Arba Kosos!

ANSWERS:

(a) When it comes to the Arba Kosos, the Rabanan decreed that one must drink the entire cup (and by drinking a majority of the cup, one is considered as if he had consumed the entire cup). In contrast, when it comes to Lechem Mishnah, there is no requirement to eat two entire loaves of bread. Just reciting ha'Motzi over Lechem Mishnah is not considered Zugos. (Y. SHAW)

(b) Our Gemara (110b) says that loaves of bread do not have a problem of Zugos because they are "completed in the hands of man." (However, some old texts of the Gemara, as well as that of Rabeinu Chananel, do not have the Girsa there of "Kikaros.")

(c) The MAHARSHAM cites the Zohar (Parshas Ekev, p. 273a) that asks this question and answers that Sheidim do not have dominion on Shabbos (although they apparently do have dominion on Yom Tov, as we see from the Gemara's question concerning the Arba Kosos).

RAV YAKOV EMDEN asks that our Gemara (112b) concludes that the Sheidim are more prevalent on Friday night than during the week! In fact, this is why we do not recite the verses normally recited before Shemoneh Esreh (in Chutz l'Aretz) on Friday night; so that we will finish Ma'ariv late and have to return home after dark, and after the lamps have gone out (TUR OC 267, citing the Ge'onim). How is this to be reconciled with the Zohar, which says that the Sheidim have no power on Shabbos?

He answers that the Zohar itself asks this question (Vayakhel 205a). The Zohar answers that the Rabim (public) have nothing to fear on Shabbos. Individuals, though, could be affected by the forces of the Sheidim on Shabbos. Since Lechem Mishnah was enacted for everyone, there is no concern for Sheidim. An individual, though, should not go out alone on Friday night, out of concern for the Sheidim that dominate then.