Gemara

(Rava): If two told Levi 'make an Eruv for me', and Levi made one Eruv before Shabbos and it was eaten Bein ha'Shemashos, and he made the other Bein ha'Shemashos and it was eaten after dark, both of them are valid.

It is a Safek whether Bein ha'Shemashos is day or night. It is reasonable to be Machshir both Eruvin.

36a - Question: R. Yosi contradicts himself! (He is lenient about Safek Eruv, and stringent about Safek Tevilah.)

Answer (Rav Huna bar Chinena): He is more stringent about Tum'ah, for it has a basis in Torah. He holds that Techumim is mid'Rabanan.

(Beraisa - R. Yosi): If one was Me'arev with Terumah, and we do not know whether it became Tamei before Shabbos or after dark, the Safek Eruv is valid;

If we do not know whether it was Tamei or Tahor from the beginning, the Safek Eruv is invalid.

(Rav Huna): We require an Eruv that one could have eaten before Shabbos.

Shabbos 34a (Mishnah): Shortly before dark [on Erev Shabbos] one must say "did you make an Eruv?"

If it is Safek dark (i.e. Safek night), one may not tithe Vadai (definite) Tevel, immerse Kelim, or light Ner Shabbos;

[At this time] one may tithe Demai (Safek Tevel), make an Eruv, and do Hatmanah (wrap food to keep it warm).

Question: The Reisha connotes that when it is Safek night, one may not make an Eruv. The Seifa permits this!

Answer (R. Aba): One may not make Eruv Techumim at this time, but one may make Eruv Chatzeros.

(Beraisa): We it (Bein ha'Shemashos) the stringencies of two days.

(Rav Huna brei d'Rav Yehoshua): This pertains to Tum'ah;

(Mishnah): If a man saw [an emission of Zivah during] one Bein ha'Shemashos, he is a Safek Zav. (Perhaps the sighting was partially during the day and partially at night.)

Rishonim

Rambam (Hilchos Eruvin 6:13): If two people told Levi 'make an Eruv for me', and Levi made one Eruv before Shabbos and it was eaten Bein ha'Shemashos, and he made the other Eruv Bein ha'Shemashos and it was eaten after dark, both of them are valid. Bein ha'Shemashos is a Safek, and a Safek Eruv is Kosher. Even so, l'Chatchilah one may not place Eruv Techumim when it is Safek dark. If he did, it is valid.

Magid Mishneh: The Rambam explains that Rava discusses Eruv Techumim, like Rashi does. They challenged this, for the Gemara says that when it is Safek dark one may be Me'arev Eruvei Chatzeros, but not Eruv Techumim! The Ramban said that perhaps this is only l'Chatchilah, but b'Di'eved it acquires, like Rashi says. It is forbidden l'Chatchilah, for one should not enter a Safek, that the beginning of the Eruv is a Safek. The Rashba questioned this, for if one was Me'arev with Terumah that was Safek Tamei, it is invalid. This shows that a Safek Eruv is Kosher only if it had a clear Chazakah of being Kosher. Therefore, an Eruv placed Bein ha'Shemashos is invalid. I say that the Rambam defended himself from this. It is invalid because it was not permitted to eat it during the day, even though the Isur is due to Safek. If it was Tahor, and it is a Safek when it became Tamei, it was permitted during the day. Such a Safek Eruv is Kosher. The same applies if one was Me'arev Bein ha'Shemashos. Eruvin 36a connotes like this. Since we hold that Techumim is mid'Rabanan, we are lenient about a Safek Eruv, even if it had no clear Chazakah, as long as there is not a Chazakah against it. We are lenient about a Safek mid'Rabanan!

Magid Mishneh: R. Chananel, the Ramban and Rashba explain that Rava discusses Eruvei Chatzeros. He said "go be Me'arev", i.e. collect the Eruv from the Bnei Chatzer. Why does the Rashba say that an Eruv Chatzeros of Terumah that was Safek Tamei is invalid? He learns from Safek Terumah that Eruv Techumim Bein ha'Shemashos is invalid. According to him, Eruvei Chatzeros disproves this! (It is Kosher Bein ha'Shemashos, even though a Safek about Tum'ah disqualifies it.) The opinion of the Rambam and Rashi is primary.

Rosh (Shabbos 2:22): The Gemara permits Eruvei Chatzeros when it is Safek night, but not Eruv Techumim. Rashi says that Rava discuss Eruv Techumim. R. Tam disagrees, for the Gemara forbids this Bein ha'Shemashos. Do not say that this is only l'Chatchilah, but b'Di'eved it acquires, like Rashi says, and it is like R. Yosi, who says that a Safek Eruv is Kosher. R. Yosi said so only when he placed it during the day, and when it was Safek day, Safek night, Bein ha'Shemashos a house caved in on it or it was burned or [it was Terumah and] it became Tamei. We leave it in its Chazakah, that it was intact Bein ha'Shemashos. If one placed it Bein ha'Shemashos, R. Yosi is not Machshir. The words 'go be Me'arev for us' forced Rashi to say that it discusses Eruv Techumim, for this connotes 'go outside the Techum (this should say "city").' We need not say so. We similarly say 'go hire workers', i.e. go from here. [Another reason for Rashi is that] 'he was Me'arev for him' connotes for an individual (i.e. Eruv Techumim). We can [reject this, and] say that he said 'take the Eruv to the house in which they place the Eruv.'

Rosh (ibid.): R. Yonah says that this is only if he made one Eruv during the day, it was eaten Bein ha'Shemashos, and he made the other Bein ha'Shemashos and it was eaten after dark. If he placed an Eruv Bein ha'Shemashos, and it was eaten Bein ha'Shemashos, it is Pasul. We are not so lenient to say that part of Bein ha'Shemashos is day, and part is night, and he placed it during the day, and it was eaten at night. We say that Bein ha'Shemashos is partially day, and partially night to be stringent, but not to be lenient, like we conclude 'we give it the stringencies of two days.'

Poskim

Shulchan Aruch (OC 393:3): If Levi made Eruvin for two people, one he made before Shabbos and it was eaten Bein ha'Shemashos, and he made the other Bein ha'Shemashos, both of them are valid. (This Siman discusses Eruvei Chatzeros.)

Beis Yosef (DH u'Mah): The Rosh holds like R. Tam, that Rava discusses only Eruvei Chatzeros. Rashi and the Rambam explain that he discusses Eruv Techumim. It seems that all the more so it is Kosher for Eruvei Chatzeros, which are more lenient.

Shulchan Aruch (ibid.): Regarding the first we consider Bein ha'Shemashos to be night, and regarding the second we consider it to be day.

Tzlach (Shabbos 34a DH Tosfos): On 76a, we asked why two Eruvin [that cannot both be Kosher] is unlike Rava's law. This shows that Rava is Machshir both even if they contradict each other, i.e. the first Eruv was eaten before the latter was placed. However, if one asked about both at once, we cannot be Machshir both, for this is a contradiction.

R. Akiva Eiger: If Levi was Me'arev for both of them in the same direction, one of them may not take the other's Peros past the Techum in that direction. If his Eruv was valid, the Eruv of the owner of the Peros was invalid. One may not take Peros where the owner may not go!

Mishnah Berurah (15): Even though they contradict each other, since Eruvei Chatzeros is mid'Rabanan, we make [favorable] assumptions to be lenient about both.

Kaf ha'Chayim (18): If one made Eruv Techumim during the day and it was eaten Bein ha'Shemashos, and then he made Eruvei Chatzeros, one of them is invalid. Since we do not know which, both are invalid. We cannot apply Safek mid'Rabanan l'Kula be lenient about both of them.

Kaf ha'Chayim (19): We do not give contradictory leniencies for one person. E.g. if he made a Safek Eruv to the west, he may not go 2000 Amos past the Techum to the west, and also [be lenient to say that the Eruv is invalid, and] go to the end of the Techum in the east.

Shulchan Aruch (ibid.): If he was Me'arev Bein ha'Shemashos and it was eaten Bein ha'Shemashos, he is forbidden.

Gra (DH Aval): Since [Rava] did not teach about this regarding one person, in such a case it is not true. (The Eruv would be invalid.)

Shulchan Aruch (415:3): If two told Levi 'make an Eruv [Techumim] for me', and Levi made one Eruv before Shabbos and it was eaten Bein ha'Shemashos, and he made the other Bein ha'Shemashos and it was eaten after dark, both of them are valid.

Beis Yosef (DH u'Mevarech): The Magid Mishneh answered all the questions against Rashi and the Rambam. The Tur rules like the Rosh, therefore he taught this only regarding Eruvei Chatzeros.

Bi'ur Halachah (DH Ne'echal): I am unsure about an Eruv eaten during Bein ha'Shemashos of R. Yehudah, which begins at sundown. Perhaps it must last until Bein ha'Shemashos of R. Yosi, which is when R. Yehudah's Bein ha'Shemashos finishes. Surely, an Eruv placed during R. Yehudah's Bein ha'Shemashos is Kosher. Letter of the law, the Halachah follows R. Yosi; until Tzeis ha'Kochavim is day (Pesachim 2a). We are stringent for R. Yehudah's opinion regarding Shabbos. We may rely on R. Yosi for Eruv Techumim.

Shulchan Aruch (ibid.): Some disagree.

Mishnah Berurah (11): This opinion is Machshir only the first Eruv [for Eruv Techumim], for it had Chezkas Kashrus. Most Rishonim hold like this opinion.

Bi'ur Halachah (DH v'Yesh): Why the Mechaber favor Rashi, the Rambam and the Magid Mishneh against nine Rishonim, i.e. R. Chananel, R. Tam, the [Tosafist] Rashba, Ramban, the Rashba, Rosh, Ritva, Ran and Mordechai?!

Note: Even though the Beis Yosef normally favors the Rambam against the Rosh, he wrote in his introduction (DH uv'Makom) "when one of these three (the Rif, Rambam and Rosh) did not reveal his opinion, and the other two argue, we will follow the majority of the Ramban, Rashba, Ran, Mordechai and Semag."

Kaf ha'Chayim (18): When the Mechaber brings an opinion Stam, and "some say..." to be stringent, l'Chatchilah one should be stringent, but in pressed circumstances one may rely on the first opinion.