When reading an Egyptian halachic work I noticed that it mentioned two customs for wrapping tefillin. The more general custom was to wrap the tefillin shel yad but leave it incomplete, put on the shel rosh, and then finish tying the shel yad. The other custom was to completely tie the yad before doing the rosh.
Since we are referring to Egypt, it seemed likely that there would be some that would be holding by the Rambam there, so upon looking in the Rambam i found the following:
When does the above apply? When one puts on only one of them. If, however, one puts them both on, one recites only a single blessing, "...to put on tefillin."
One should tie the tefillin on one's arm, and afterwards put on the head tefillin. When one removes them, the head tefillin should be removed, and then the arm tefillin.
The Rambam seems clear that one puts on the arm [completely] and then puts on the head. So where does the custom of leaving the yad incomplete come from? Is it based on kabbalah?