10:8 One may not interrupt between putting on the hand tefillin and the head tefillin with talking. He may not even communicate by gesturing or winking. (Preferably, one should not even interrupt with a long, silent pause – Mishnah Brurah 25:29.) The Torah tells us (Exodus 13:9) that the tefillin are to serve as a sign upon our hands and a memorial between our eyes, so it is only appropriate to put them on immediately following one another in order that we should treat them as a single matter. One may not even interrupt putting on his tefillin in order to answer Kaddish or Kedusha. Rather, he must pause and silently listen to the prayer and response. One can, however, reply “Amen” to another person reciting the bracha l’haniach tefillin (that God commanded us to put on tefillin), since that is relevant to his own activity and does not constitute an interruption. Those who wear Rabbeinu Tam tefillin in addition to Rashi tefillin may not interrupt putting them on by speaking, but they may respond to Kaddish and Kedusha. (Mishnah Brurah 25:37 says that in the case of Rabbeinu Tam tefillin interrupted for Kaddish or Kedusha, a person should re-adjust his arm tefillin, similar to what we will discuss immediately in 10:9.)
10:9 If a person made a mistake and interrupted in between putting on his head and hand tefillin, he should touch his hand tefillin and repeat the blessing of l’haniach tefillin. He then tightens the knot, puts on the head tefillin and says the blessing al mitzvas tefillin. If the interruption was for the sake of performing the mitzvah, he does not repeat the bracha l’haniach tefillin.