Question: When I put on Rabbeinu Tam tefillin (at the same time as Rashi tefillin), should the retzu’os be positioned with one set of retzu’os on top of the other, or should they be adjacent to each other?
Answer: Firstly, regarding the placement of the batim and retzu’os of Rabbeinu Tam tefillin, many individuals who put on two pairs of tefillin together do so in a way that the retzu’os of Rabbeinu Tam are covered by the retzu’os of Rashi (i.e., effectively there will be two layers of retzu’os around one’s head, with Rabbeinu Tam under Rashi).
However, Rav Mordechai Eliyahu (Ma’amar Mordechai – Halachos Le’ymos Hachol Perek 8 Se’if 49) maintained that it is more ideal that the retzu’os should be placed on the head in a way that the two sets of retzu’os are not one on top of the other, rather both should be placed directly on the head (because of concerns of chatzitzoh) adjacent to each other. This pertains to the retzu’os around the head, but around the area of the kesher inevitably there will be some overlap, and the retzu’os that hang from the body should also hang together with the Rabbeinu Tam retzu’os under Rashi retzu’os.