If one accidentally placed on his Shel Rosh prior to his Shel Yad, is he to remove his Shel Rosh prior to wearing the Shel Yad?[1]

Some Poskim[2] rule that he is required to first remove his Shel Rosh and only then is he to put on his Shel Yad and then wear his Shel Rosh. Other Poskim[3] rule that he may put on his Shel Yad without removing his Shel Rosh. Practically, one is not to remove his Shel Rosh prior to putting on his Shel Yad.[4] If, however, he has not yet said the blessing over the Tefillin, then he may remove the Shel Rosh and then wear the Shel Yad.[5] See Chapter 3 Halacha 2C regarding if the blessing of Lehaniach was said over the Shel Rosh.

[1] See Shaareiy Teshuvah 25:13; Biur Halacha 25:5 “Paga”; Kaf Hachaim 26:1; Divrei Shlomo 2:114; Afrakasta Deanya 2:1; Miasef Lekol Hamachanos 25:93; See Likkutei Sichos 19:47, brought in Shulchan Menachem 1:72-80 that the dispute is dependent on the understanding of why the order of first wearing the hand Tefillin was given, and if it is a law on the actual Tefillin or a law on the person, which is a dispute between the Talmud and Mechilta.

[2] M”A 684:5; Abudarham, brought in Taz 684:4; Halachos Ketanos 1:53; Artzos Hachaim 25; Shulchan Hatahor 25:10; Rav Poalim 1:4; Sefer Hachaim 25; Shevet Halevi 3:7; Poskim brought in M”B 25:23

[3] Taz 684:4; Elya Raba 684:2; Birkeiy Yosef 26:1 in name of Rav Malko; Kesher Guda 3:3; Shiyurei Bracha 25:3; Sheilas Yaavetz 125; Beis Shearim 1:23; Shaareiy Teshuvah 25:13; Rav Akiva Eigar 25; Peri Megadim 25 A”A 11; 684 M”Z 4 in name of Elya Raba; Maharshag 1:9; M”B 25:22; Biur Halacha 25:6 “Paga”; Poskim brought in Kaf Hachaim 26:1; Ketzos Hashulchan 8:6

[4] M”B ibid; Kaf Hachaim ibid

[5] P”M 25 A”A 7 and 11