The Rambam (Hilchos Bechoros 8:10) writes, "If four left from opening 1 and then four left from opening 2, and six remained... if the last six left from the two openings, the six join for the next Goren, and the first eight are exempt, for each four were part of a Minyan ha'Ra'uy...."
The Lechem Mishneh explains 'the last six left from the two openings', i.e. some left from each.
If so, according to the Rambam, why do all six join for the next Goren? The first of the six that left should be exempt due to Minyan ha'Ra'uy, for the other five could have left through the same opening!
The Lechem Mishneh says that the Rambam's Perush is better than Rashi (who explains that they left through a third opening). I noticed that Sefer Mafte'ach in the Frankel Rambam cites Bnei David (I have no access to this Sefer), who says that Rashi's Peshat is Muchrach. In any case, how is it that none of the commentaries were bothered by this?! Is there another way to explain the Rambam unlike the Lechem Mishneh?
Pesach Feldman, Yerushalayim
1. The Bnei David answers with the explanation of the newly printed Shitah Mekubetzes. I have not found this Sefer but the explanation that Rav Greinemann cites appears to be the same as the one given in Rashi Kesav Yad DH v'Im Lav (printed in the margin of the Gemara).
2. Rashi Kesav Yad writes that the last six animals were split into two groups of three animals each, and they started going out simultaneously from the two openings.
If we say that the Rambam agrees with this Peshat in Rashi (of course, this is not the same Peshat as that of the Rashi which the Lechem Mishneh cites), then we can answer your questions. In fact, one does not have to say that all six went out b'Bas Achas. It is sufficient that the first two went out simultaneously. This means that the when the first of the six went out, there were no longer five left in the "Dir" to make a Minyan ha'Ra'uy, because one has already gone in the direction of the other opening and can no longer join up with the five that might still go out of the same opening.
3. Chidushim u'Vi'urim adds that b'Bas Achas does not have to mean that the animals literally went out simultaneously. Rather, it is sufficient that the second animal put its head out of the second opening before the head and the majority of the body of the first animal left the first opening. According to this explanation, there is no problem of "Iy Efshar l'Tzamtzem" -- that it is impossible to be so precise that both animals should leave the respective openings at exactly the same time.
Kol Tuv,
Dovid Bloom