Answering Kaddish between the Arm and Head Tefillin

Hi,

One may not respond to Kaddish between donning his arm and head tefillin; rather, he should be silent and listen, and respond only mentally.

One who normally recites separate blessings on the arm and head tefillin, and who forgets and responds to Kaddish between his arm and head tefillin, should only recite the normal blessing for the head tefillin.

(Rav Moshe Feinstein, Igrot Moshe Orach Chaim 2:10)

Have a great day,
Mordechai