How One Blessing Over Tefillin Became Two Blessings

With the recent incident of tefillin in the news from last week (and Bangitout's piece), I thought it apropos to mention something I read a couple of weeks ago. In Aaron Amit, "The Curious Case of Tefillin: A Study in Ritual Blessings," Jewish Studies Quarterly 15, no. 4 (2008): 269-288, Amit explores how/why there are either two blessings recited over tefillin or just one. I'll just skip to some of his concluding remarks (although I highly encourage reading it in its entirety):

Initially, the accepted practice in Babylonia was to recite one blessing on the tefillin after both were in place. This is clear from the statement attributed to Samuel (among others): תפילין - מאימתי מברך עליהן? משעת הנחתן. Likewise, the early Babylonian amoraim knew of only one blessing on the tefillin. Therefore, when Rav Hisda addressed the case of interruption, he used the words חוזר ומברך, one must return and bless again, i.e. repeat the blessing. In both cases, it is the Babylonian amoraim Abaye and Rava, who, aware of Palestinian customs that differed from the Babylonian, create explanations which allow for both rulings and meld them into a coherent whole. They reinterpret Samuel's words as referring to the point at which the tefillah shel yad is placed on the arm but before it is tied. Likewise, they reinterpret Ravi Hisda's law regarding interruption in a manner which allows for the recitation of the second blessing mentioned by Rabbi Yohanan under certain circumstances. Abaye and Rava are responsible for an editorial process which attempts to harmonize Babylonian and Palestinian rulings. This sugya serves as an example of how the process of redaction which culminated in the Talmud as we know it, a product of post-amoraic Babylonia, actually began in amoraic times. (p. 286)

Amit moves beyond the Talmud to discuss later rabbinic codification on pp. 286-287:

Rabbi Joseph Karo in Shulhan Arukh Orah Hayyim 25:5 mandates only one blessing, to be recited on the donning of the tefillah shel yad, arguing that the second blessing is only necessary in the event that there is an interruption between donning the tefillah shel yad and tefillah shel rosh. This is in keeping with original tannaitic and amoraic practice as described above, and with Abaye and Rava's ruling in the Bavli according to the most plausible reading.
Rabbi Moses Isserles, on the other hand, in his gloss on that passage, requires two blessings on the donning of tefillin: on the tefillah shel yad, one recites אשר קידשנו במצותיו וציונו להניח תפילין and on the the tefillah shel rosh אשר קידשנו במצותיו וציונו על מצות תפילין. This is in keeping with the practice instituted by Rabbi Yohanan's disciples in the wake of other practices and rulings of Rabbi Yohanan, and it is also the practice attributed to Rabbi Yohanan himself by the Bavli. It also reflects Abaye and Rava's ruling as understood by Rabbenu Tam.
Isserles concludes his gloss with the words: "and it is good to always add after the second blessing: ברוך שם כבוד מלכותו לעולם ועד". This formula usually serves to annul an inappropriate blessing recited accidentally. It should be noted that the use of the Barukh shem formula here is not accepted by all authorities. For example, Rabbi Isaiah Horowitz argues that one may not employ the Barukh shem formula when the blessing was recited intentionally.