Although the Torah commands that we put tefillin on both the arm and the head, and that we do them in one continuous act, as we have learned, that does not mean that if one puts on only one ‘tefilah’, he has not accomplished anything. In fact, putting on each ‘tefilah’ fulfills one distinct mitzvah. This is true whether one has only one, or is able to put on only one!
Question: Which bracha/os would one say (i.e. Ashkenazi) if he was donning only the ‘shel yad’? Only the ‘shel rosh’? Answer: If it is the ‘shel yad’, he should say only the bracha which ends ‘l’hani-ach tefillin’. If it is the ‘shel rosh’, he should say both brachos - ‘l’hani-ach tefillin’ and ‘al mitzvas tefillin’. (See above for a more thorough discussion of the brachos of tefillin.)