הלכה על מנחות 71:12

One places the Shel Yad first, and after that the Shel Rosh. And even if one touches the Shel Rosh first — he is forced to pass on it, even though we don't pass up on the performance of Mitzvot, and he should take the Shel Yad and place it on his arm, and afterwards should take the Shel Rosh and place it on his head. And the reason is because it is written: "And they shall be a Totafot between your eyes", and we learn out (Menachot 36a) from the language of "they" since it is in the plural (Rashi), that all the time that it [the Shel Rosh] is "between your eyes" — there should be two [both the Shel Rosh and the Shel Yad]. And even though that if one doesn't have the Shel Yad, or he is unable to place the Shel Yad, he [still] places the Shel Rosh alone as I have written in Chapter 26 — this is because they are two Mitzvot, and of course he is obligated to fulfill whatever he can. But when he has both [and can place both] — it is a decree of the Torah to place the Shel Rosh after the Shel Yad, and thus we are not concerned with "we do not pass up on the performance of Mitzvot". And thus Our Rabbis of Blessed Memory said (Yoma 33a) that one should be careful to not touch the Shel Rosh first, so that he will not have to pass up on the performance of the Mitzvah.