Rabeinu Tam tefillin
1. We have already explained, in chapter 22:1, the order in which the portions are inserted into the shel rosh: kadesh, ve-haya ki, shema, ve-haya im, and they have this order in the shel yad also. This is the opinion of Rashi and the Rambam, and this is the universal custom. However, Rabeinu Tam had a different opinion, namely that kadesh should be on the wearer’s left, and then ve-haya ki, ve-haya im in the third compartment and shema in the outer compartment on the right-hand side, and in the same order in the shel yad – kadesh, ve-haya ki, ve-haya im, Shema.
2. Even though the universal custom is to follow Rashi and Rambam, the very pious observe both opinions, making and laying two pairs of tefillin. They intend that when they lay them, whichever one is the correct one shall be the one with which they do the mitzvah, and the other one shall be merely a strap, as the sites on the head and arm where the tefillin are laid are big enough for both pairs – provided the tefillin, including titura and ma-abarta, measure less than two fingers’-width, as the correct site on the head measures only four fingers’-width (Maḥatzit ha-Shekel). If one does not know how to locate the correct site such as to lay both pairs at once, it is the custom to lay Rashi, make the blessing, and daven in them, and after davening to lay Rabeinu Tam with no blessing, and to read the Shema and Ve-haya im (but he need not read Vayomer in them – Ateret Zekenim).
3. Rabeinu Tam concurs that they must be written in the order in which they appear in Torah. Therefore, in the shel yad, where they are all on one piece of klaf, he leaves enough space to write Ve-haya im before writing Shema, and then writes Shema and afterwards goes back and writes Ve-haya im. At the end of Ve-haya im he must leave a space the size of nine letters, so that Shema, after it, will be petuḥa, just as it is petuḥa in Torah. The portion Ve-haya im is done differently in Rashi tefillin – space is left so as to make it setuma, as explained in chapter 20:7 – and it is also done this way in Rabeinu Tam tefillin. That is, he leaves a space of less than nine letters (yuds) at the end of Ve-haya ki, and another space of less than nine letters (yuds) at the beginning of Ve-haya im, such that the two spaces together are nine letters (yuds) long.
4. One may not put the two pairs in the same bag, since one of the pairs is not holy, and may not be placed in a tefillin bag; instead he should have two bags, labelled so that they will not get swapped around. All the more so is he prohibited from swapping the straps, housings or portions (R’ Menaḥem Azariyah). One may not label the tefillin themselves so as to tell which is Rashi and which Rabeinu Tam; he should find other features of its appearance by which to tell them apart. Clear differences such as the relative sizes of the housings or the straps are not inferior (ibid. ch. 38)
5. If one found a strap but didn’t know whether it was from Rashi tefillin or Rabeinu Tam tefillin, in these lands he should assume it is a Rashi strap, since the huge majority of people wear Rashi tefillin only. Accordingly, he may not put it on Rabeinu Tam tefillin.