אין מערבין עירובי חצירו' ושיתופי מבואו' בי"ט שחל להיות בע"ש ואם שכח ולא עירב מערב יום טוב וחלו שני ימים טובים ביום החמישי ויום ששי יערב בי"ט על תנאי שיאמר אם היום חול יהא זה עירוב ואם היום קודש אין בדברי כלום ולמחר יאמר אם היום קודש הרי עירבתי מאתמול ואם היום חול יהא זה עירוב והני מילי בשני י"ט של גליות אבל בשני ימים טובים של ר"ה לא דכיומא אריכתא דמי (ועיין לקמן סי' תקכ"ח):

One does not make eruv chatzeirot (courtyard eruv) or shitufei mevoot (alleyway partnership) on a festival that falls on Friday. But if one forgot and did not make an eruv before the festival, and two festival days fall on Thursday and Friday, one may make an eruv on the festival conditionally, saying: "If today is a weekday, let this be an eruv; if today is holy, my words are nothing." And the next day one says: "If today is holy, I already made an eruv yesterday; if today is a weekday, let this be an eruv." This applies to the two festival days observed in the diaspora, but not to the two days of Rosh Hashanah, for they are considered like one long day. (See below, section 528.)

Both eruv chatzeirot and shitufei mevoot may be established during twilight, even if one has already accepted the addition to Shabbat. Some authorities forbid this if one has already accepted the addition to Shabbat.

If one made an eruv for two people—for one person while it was still day and that eruv was consumed during twilight, and for another person during twilight—both have acquired their eruv. For the one whose eruv was consumed during twilight, we consider it night; for the one whose eruv was placed during twilight, we consider it day. However, if one made an eruv during twilight and it was consumed during twilight, it is forbidden.