Siman 26

Tefillin of the hand and of the head are two mitzvot. The Rambam and all those who count the mitzvot counted them as two positive mitzvot. And so we learned at the beginning of the chapter "HaTekhelet": Tefillin of the hand does not prevent that of the head, and that of the head does not prevent that of the hand. And even though regarding tzitzit, the tekhelet does not prevent the white, and the white does not prevent the tekhelet, and nevertheless they are one mitzvah—this is because they are one act (Rivash siman 137). That is to say: it is like a mitzvah that contains parts of the mitzvah, and the mitzvah is tzitzit, and the parts of the mitzvah are tekhelet and white. But the tefillin are two acts, each one on its own.

Therefore, the Tur and the Shulchan Aruch wrote that one who has only one tefillah—he dons the one he has and recites the blessing over it, for each one is a mitzvah unto itself. And the same law applies if he has both of them, but has some constraint such that he can only don one—he dons the one that he can. Thus far their words. And the Tur added to explain: for example, if he needs to set out on a journey and cannot delay—he may don one of them… Thus far his words. And it appears that he informs us of a novel point: not only when he has a wound on the arm or on the head, such that he cannot don one of them, that he dons the one that he can; but even if he can don both, but the haste of the journey hinders him, and I would have said that in such a case they did not permit it; and certainly if he dons the of-the-head alone, for the Torah said as long as it is "between your eyes" there shall be two—he informs us that there is no concern, for since he cannot don the of-the-arm now, he may don the of-the-head alone. And if there is not enough time to don both but it is within his power to choose whichever he wants—he should don the of-the-arm alone so as not to change the order in the Torah (Artzot HaChayim). And there is one who does not say so, but it is not the primary view. And know that all this is when he will don the second one on the journey. But if he will not don it at all—certainly they did not permit him because of the haste of the journey to be nullified from the mitzvah of tefillah as is proper. Unless he is going for the saving of a life or such a case, for one who is occupied with a mitzvah is exempt from a mitzvah. But in another manner they did not permit it. (And so it is implied from Magen Avraham, subsection 1, see there. And examine carefully and you will find it simple.)

If one dons the shel yad alone – he should recite the blessing "le-haniach." And if the shel rosh alone – he should recite the blessing "al mitzvat." Our teacher, the Rema, wrote that for us, who are accustomed every day to recite two blessings – he should recite two blessings over the shel rosh alone. And if he dons the shel yad alone – he recites "le-haniach" alone. Thus far his words. And so wrote the Tur, see there. And there are those who hold that even over the shel yad he recites two blessings. But the primary view is as I have written, and so the great Acharonim have agreed.

It is a simple matter that if one needs to pray and has only one tefillin—he should don the one he has in order to pray with the tzibbur, and all the more so if the time for the recitation of Shema will pass. For he should not wait for the second one, as there is no "false testimony" here since he does not have it. And it appears more likely to me that if the time for the recitation of Shema will pass and he does not have tefillin now, or he is unable to don them, such as if he is on a journey and the cold is great—he should recite Shema without tefillin since he is under duress, and afterward he should don them and recite Shema once more after the time. And so it appears to me that he may pray with the tzibbur if the time for prayer will pass, and don them afterward.

One who erred and donned the shel rosh before the shel yad, they wrote in the name of the Abudarham that he should remove it from his head, and don the shel yad and afterward the shel rosh (Arzot HaChayim). However, if he recited the blessing over the shel rosh – he should not recite the blessing again. And one who has no arm at all, in a manner that he will never don the shel yad, nevertheless he is obligated to don the shel rosh alone and recite the blessing over it (Sha'agat Aryeh siman 34).