Maaseh Rav Siman 22
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סח מברך שתים לא סח מברך אחת כפי' רש"י ז"ל דאם לא סח יברך על של יד ועל של ראש אחת להניח ואם סח יברך על ש"ר על מצות תפלין:
Sefaria Community Translation
If one spoke (between putting on arm and head tefillin) he recites two blessings, and if he did not speak, he recites one blessing, in accordance with Rashi of blessed memory. For if he did not speak he recites on both hand and head one (blessing of) 'to place tefillin', but if he spoke, he recites on the head (a separate blessing of) 'on the mitzva of tefillin'.