The Mitzvah of wearing Tefillin:
It is a positive command in the Torah to wear Tefillin. It incorporates two positive commands, one to wear the head Tefillin and the second to wear the hand Tefillin. This command is repeated in the Torah 4 different times, and hence one who does not wear Tefillin transgresses 8 positive commands.
One who only has one Tefillin, Shel Yad or Shel Rosh: If one only has one Tefillin, whether the Shel Yad or the Shel Rosh, he is to wear the Tefillin that is available. A blessing is to be recited over the Tefillin. The same applies in a case that one has both Tefillin’s but is unable to wear both of them due to reasons beyond his control [i.e. a head or arm injury], nevertheless he is to wear whatever Tefillin he is able, and recite a blessing over it. In a case that one is only wearing the Shel Yad Tefillin he is to say the blessing of Lehaniach. If one is only wearing the Shel Rosh, only the blessing of Al Mitzvas Tefillin is recited.
 The head Tefillin and arm Tefillin are listed as two separate Mitzvos of the 613. [Sefer Hamitzvos of Rambam Mitzvas 13 and 14]
 Menachos 44a; Rambam ibid
 Admur 26/1; Michaber 26/1; Mishneh Menachos 32a; Ketzos Hashulchan 8/20
 The reason: As each Tefillin is a separate Mitzvah and the two Tefillin do not invalidate eachother when not worn together. [Admur ibid]
 Admur 26/2; Michaber ibid
 Admur 26/5; Rama 26/2; This applies according to all.
 Admur ibid according to second opinion; Siddur Admur who rules like this opinion
Other opinions in Admur: The Ashkenazi custom is to recite two blesisngs when wearing the Shel Rosh even if he will not be wearing the Shel Yad. These blessings are 1) Lehaniach Tefillin and b) Al Mitzvas Tefillin. [Admur ibid and 25/23; Rama 25]