Is there a typo in the Tosfos DH mei'Aimasai. When the Tosfos say,
v'Im Tomar Heich Anu Mispalilim Tefilas Minchah Samuch l'Minchah ... mi'Kol Makom Kashia ... she'Harei mei'Eizeh Ta'am Anu Mispalilim Arvis mi'Yad l'Achar Plag ha'Minchah.
It seems that the word Plag in the last line is out of place.
Tosfos seems to be asking why do we daven Mincha & Ma'ariv just before nightfall? We say Ma'arive right after Mincha (not right after "Plag Mincha as in the printed text) because we follow R. Yehuda that after Mincha we can daven Ma'ariv, but when we daven Miincha we don't follow R. Yehuda.
The text as printed is difficult because it assumes we say Mincha close to nightfall but then says we daven Ma'ariv right after Plag HaMincha, which would have been earlier.
Steve Friedell, Philadelphia PA USA
The Tosfos is precise as written. First, he says that one may recite Ma'ariv immediately after Plag, like Rebbi Yehudah. He then discusses the reason why we may still recite Minchah after Plag even though it is considered nighttime for Keri'as Shema. He explains that we rule like the Rabanan that the end of the day is at Sheki'ah. However, if this is how we rule, then how can we recite after Plah, and before Sheki'ah, both Ma'ariv (considering it night) and Minchah (considering it day)? These are two contradictory leniencies, the leniency of Rebbi Yehudah (to recite Ma'ariv after Plag) and the leniency of the Rabanan (to recite Minchah until Sheki'ah)! He is not discussing our custom today of doing both together; he is merely pointing out that we cannot one time recite Minchah after Plag and another time call it night and recite Ma'ariv at that time.
The Halachah, however, is that one may do this as long as he is not doing it on the same day, in which case one should recite Minchah before Plag and Ma'ariv after Plag, according to the Rema. According to the Shulchan Aruch, one may recite both after Plag and the contradiction does not matter.
Yoel Domb