I have recently seen a Chasid wearing two baatim (Rashi and Rabeeinu Tam) on his head but only (Rashi) for Tefillin Shel Yad. Later, a third Tefillin Shel Rosh were applied but the Shel Yad stayed put. Is this halachically correct?
Welcome to mi.yodeya, Aharon Palmer!– ezraNov 1, 2017 at 15:17
Let us continue this discussion in chat.– ezraNov 1, 2017 at 15:23
1Aharon, could you clarify something for us? Are you sure he was a chasid? What Chasidic Dynasty was he from? Did he wear three shel roshes at the same time?– ezraNov 1, 2017 at 17:54
I have never heard of such a practice and your best bet is to ask the person himself. there are those who wear more than just rashi and rabenu tam but this is not how that is described at all– Laser123Nov 1, 2017 at 17:58
I can't recall details but besides for the more common rashi and rabeinu tam Tefilen there is also of the rai'vid and a fourth who's name I can't remember. These last 2 have one bayis being the same as one of the other 2 opinions. So if someone was accustomed to fulfilling Tefilen according to all opinions he would put on rashi take of one bayis, replace it with with opinion 3. Take everything off. Put on rabeinu tam Tefilen. Take one bayis off and replace it with opinion 4. I'm sure some of my details are wrong but that's the general idea.– mrollNov 2, 2017 at 22:00
This practice is halachicly justifiable, although I've never heard of people doing this in practice.
The Shulchan Aruch when bringing the machlokes about the order of the parshios brings from the Rosh and other Ashkinazi poskim to have two tefilin shel yad, one in each order. However, it is not so clear that the order of the shel yad changes according to Rabainu Tam. The Rambam in a teauvah (brought by the kesef Mishnah maybe?) brings the order of the shel yad as a proof to the rashi order against RT.
A third order for the parshios is the Shimusha Raba, which is the reverse of Rashi. There is an opinion, I believe the Ravad, that is the reverse of RT, but the Arizal was machmir for SR and not this fourth order. In the shel yad, the SR holds like Rashi, and the fourth order can hold like RT, which as I said is uncertain. The Ari put on SR tefilin at mincha, but why not after shacharis?
Some have the custom of wearing both at the same time. The ou site has a discussion concerning Rashi and Rabbeinu Tam tefillin. See point 4.