שולחן ערוך אורח חיים · סימן כ״ו

English

The Laws Regarding One Who Has Only One Tefilla. Containing 2 Se'ifim If one has only one tefilla, he should put on the one he has and recite the blessing on it, for each tefilla individually is its own mitzvah. And the same applies if he has both of them but there is some unforeseen circumstance where he is only able to put on one - [in that case,] he should put on the one that he is able to.

English

If he is only putting on the head tefillin, he should recite the blessing "...on the mitzvah of tefillin" only. Rem"a: And for us who have the custom to say two blessings over tefillin each day, one should recite the two blessings on the head-tefillin. And if one only puts on the hand tefillin, he should only recite the blessing "...to place" (Tur).