Gemara

40b (Mishnah): On Yom Kipur, the Kohen Gadol reads "Acharei Mos" and "be'Asor" from the Torah. He reads by heart the Parashah in Sefer Bamidbar.

41 (Reish Lakish): They do not bring a second Sefer Torah, for this would case an unnecessary Berachah (on reading from the second Sefer).

44a (Mishnah - R. Yosi ha'Glili): Even if one transgressed a Mitzvah mid'Rabanan, he returns (with fearful people) and does not fight in war.

(Beraisa): If one talked in between putting on the Tefilin Shel Yad (of the hand) and the Shel Rosh (of the head), this is an Aveirah, and he returns.

This is like R. Yosi ha'Glili.

Menachos 36a (Rav Chisda): If one talked in between Tefilin Shel Yad and Shel Rosh, he returns and blesses again.

Inference: He blesses again only because he interrupted.

Question: R. Chiya brei d'Rav Huna taught that (normally) one blesses 'Lehani'ach Tefilin' on Shel Yad, and (only) 'Al Mitzvas Tefilin' on Shel Rosh!

Answer (Abaye and Rava): If he did not interrupt, he blesses once. If he interrupted, he blesses twice.

Rishonim

Rif and Rosh (Rosh Hashanah 11a and 4:12): The Rosh Yeshiva answered that if one talked after the first Teki'os, we rebuke him, but he does not bless again before the Teki'os of Musaf. This is unlike one who talks between Tefilin Shel Yad and Shel Rosh, who blesses again. Those are two Mitzvos. One is not Me'akev the other. Also, one who interrupts says a different Berachah on the Shel Rosh, Al Mitzvas Tefilin. Even so, he returns from war for this Aveirah. Shofar is like Hallel. We bless on it, but one who talked between Perakim does not bless again.

Rebuttal (Ba'al ha'Ma'or Sof 12a): Why is he rebuked? Tefilin is unlike all Berachos and Mitzvos. "They will be a sign on your hand, and a commemoration between your eyes" - these beings (the Shel Rosh between your eyes, and the Shel Yad on your hand) must be in immediate succession. Therefore, one who interrupts blesses again on the Shel Yad, moves (and returns) it and tightens the knot, and blesses a second Berachah on Shel Rosh, so the two Mitzvos are adjacent. It is as if he blesses twice on the Shel Rosh. The early Ge'onim and all Chachamim of France conduct like this.

Ran (DH Sho'alu): A Hefsek between Tefilin obligates another Berachah, but talking before Teki'os does not. We never heard that one who blessed on a Mitzvah, such as Bedikas Chametz, may not talk until finishing! Is one who blessed ha'Motzi forbidden to talk until he finishes eating?! However, since the Rosh Yeshiva said this, we should be stringent.

Rosh (Chulin 6:6): If one talked between slaughtering one bird and another, he blesses again, like regarding Tefilin. Perhaps it is a bigger Aveirah regarding Tefilin, for it is one Mitzvah. Shechitah is optional. He slaughters more only if he wants. However, one may not cause an extra Berachah. The Kohen Gadol reads verses by heart; they do not bring another Sefer, to avoid an extra Berachah. Alternatively, perhaps Shechitah is like a meal; one who talked in the middle need not bless again. Tefilin is different, for it is one Mitzvah.

Tosfos (Chulin 87a DH u'Mechasi): It is not clear whether or not a Hefsek (interruption) between Shechitos obligates another Berachah. Surely, talking about matters of Shechitah does not interrupt. If a Hefsek obligates a new Berachah, surely it is forbidden, like regarding Tefilin. Really, one cannot prove this from Tefilin, for it is one Mitzvah, and he should not interrupt it.

Rambam (Hilchos Tefilin 4:6): If one blessed 'Lehani'ach Tefilin' and tied the hand Tefilin, he may not talk before putting on the head Tefilin, even to return Shalom to his Rebbi. If he talked, this is an Aveirah. He must bless a second Berachah 'Al Mitzvas Tefilin', and then put on the head Tefilin.

Rosh (Hilchos Tefilin Siman 14): Rashi explains that if he did not interrupt, he blesses once for both of them. If he interrupted, he blesses twice: he already said Lehani'ach on the Shel Yad, and now he says 'Al Mitzvas Tefilin' on Shel Rosh. R. Tam explains that if he did not interrupt, he blesses once on the Shel Rosh 'Al Mitzvas. If he interrupted, he blesses twice on it, i.e. he also repeats 'Lehani'ach.' Shimusha Rabah agrees. This is more reasonable. If we enacted a Berachah on Shel Rosh alone (if he interrupted after the Shel Yad), why shouldn't one say the Berachah when putting on Shel Rosh and Shel Yad? If Lehani'ach exempts also Shel Rosh, he should bless Lehani'ach also when blessing on Shel Rosh alone!

Note: We bless 'she'Kocho u'Gvuraso...' on thunder, and 'Oseh Ma'ase Bereishis' on lightening, yet either Berachah can exempt both! (Mishnah Berurah 227:5)!

Rosh (ibid.): Rather, we enacted to bless Lehani'ach first, and this applies also to Shel Rosh. When he puts on Shel Rosh, he blesses 'Al Mitzvas', for this complete the Mitzvah. Therefore, if he interrupted, he must bless both on the Shel Rosh. The same applies if he puts on only the Shel Rosh. 'He returns and blesses' connotes that he repeats what he already said. The questioner thought that he makes a different Berachah; Abaye and Rava answered that he repeats the Berachah he said. BaHaG, Rav Amram Gaon and most Ge'onim hold like this. The Rif, Rav Hai Gaon and Rav Acha hold like Rashi. In my youth, I made one Berachah, like my brother said in the name of R. Shmuel of Ivra. After I learned Menachos, I make two Berachos; Maharam me'Rotenburg also did so.

Poskim

Shulchan Aruch (25:9): One may not interrupt by talking in between the Shel Yad and the Shel Rosh. If he did, he blesses on the Shel Rosh 'Al Mitzvas Tefilin.'

Kaf ha'Chayim (58): If he interrupted while the Tefilin was over his head but before it touched his head, this obligates a new Berachah.

Rema: We normally bless two Berachos even if he did not interrupt. Therefore, if he interrupted, he blesses on the head Tefilin 'Lehani'ach' and also 'Al Mitzvas.

Rebuttal (Gra 17): The Medrash Tanchuma (Bo 14) is like Tosfos. We must say that Rashi's text said 'he blesses', without 'returns'. However, a Tosefta (Berachos 6:15) says like Rashi. Also, one who separates Terumah and Ma'aser blesses only once (Tosefta 6:19, and YD 331:78). If he interrupted, he says the proper Berachah for each. The Rema did not argue there. One cannot bless "v'Tzivanu" twice on one Mitzvah!

Defense (Mishmeres Chayim): One cause (the produce is Tevel) obligates both Terumah and Ma'aser, therefore one Berachah suffices. Here, each Tefilah (head and arm) is an independent obligation!

Magen Avraham (15): When one repeats the Berachah on the Shel Yad, he moves it (from its place) and tightens the knot Ran (11a DH Sho'alu). This is because some say that moving Tefilin obligates another Berachah, and through this he fulfills the opinion that does not obligate repeating Lehani'ach.

Rebuttal (R. Akiva Eiger): This does not fulfill Rashi's opinion that Lehani'ach suffices for both Tefilin! Rather, to fulfill both opinions he blesses Al Mitzvas on the Shel Rosh, then moves the Shel Yad and blesses Lehani'ach.

Mishnah Berurah (32): If he blessed without moving it, he moves it now.