הלכה על מנחות 70:15

And regarding blessings [on the Tefillin] there is a dispute among the Rabbis. There are those from our rabbis who ruled that one should make one single blessing upon both of them. And there are those who ruled that one should make two blessings: On the Shel Yad, "To place the Tefillin" and on the Shel Rosh, "About the Mitzvah of Tefillin". And the Ashkenazim follow that latter opinion, and the Sefardim mke one blessing, like the first opinion. And it is simple in the Gemara in Menachot (36a) that: If one didn't converse between the [placing of the] Shel Yad and the Shel Rosh — makes one blessing. [If he] conversed — makes two blessings. This implies that there is one blessing upon both, and so explains Rashi. However, it is simple in the Gemara of Berachot (60b) that says: When one places the Tefillin upon his arm, he should say, "Blessed...to place...", when he places [the Tefillin] upon his head he should say "Blessed...about the Mitzvah..." — it is clear that he needs two blessings[!] And so it is in the Yerushalmi, and in the Midrash Tanchuma at the end of Parashat "Bo", see there. And so ruled Rabbeinu Tam, and explained that [what is written] in Menachot, that "if he converses — he makes two blessings" on the Shel Rosh, alone. And Rashi would explain that the blessings [found in the Gemara Berachot 60b] are when he conversed. And the Rif and the Rambam and the Rashba ruled like Rashi. And the Rosh and the Tur ruled like Rabbeinu Tam. (And it is not clear according to Rabbeinu Tam regarding the "if he conversed — he makes two blessings" upon the Shel Rosh, yet how can one make two blessings upon one Mitzvah? And the Maharal from Prague was already awakened to this matter which was brought down in the Bach, and he wrote that one should move around the Shel Yad [when saying one of the two blessings had he conversed], and thus one of the blessings would be upon the Shel Yad [thus it would not be two blessings on one Mitzvah, but rather one on each Tefillin]. And according to this when he has only the Shel Rosh — he should only make one blessing, see there. But this is not implied from Tosfot and the Rosh, but it is implied from the Magen Avraham. And I have also found in the Or Zarua Gadol Chapter 582 in the name of the Geonim that technically each Tefillin would require two blessings: "to place" and "about the Mitzvah of", see there. And it seems to me that "to place" is the blessing upon the Mitzvah, and "about the Mitzvah of" is the blessing of gratitude that God merited us to make a sign between God and us. And this is related to the Shel Rosh, like they learned out (Menachot 35b) on the verse "they will all see...that the name of God is called upon you and they will be in awe of you" — this is [referring to] the Tefillin Shel Rosh. And according to this it is simple. And if you investigate you will find that it is simple.)