By Rabbi Berach Steinfeld
Yehudah put on his tefillin one morning as he always did and upon finishing davening he noticed that he had accidentally put on his Rabbeinu Tam tefillin instead of his Rashi tefillin. Yehudah quickly took off the Rabbeinu Tam tefillin and as he was ready to put on the Rashi tefillin realized that he has a dilemma. Should he make a new bracha on the Rashi tefillin, or is the bracha that was made on the Rabbeinu Tam tefillin sufficient to cover the Rashi teffilin?
The question here is twofold. Firstly, is the bracha that he originally made with the intent to be yotzei both the Rashi and Rabbeinu Tam tefillin? Secondly, did the act of putting on the Rabbeinu Tam tefillin cause a hefsek after the bracha was made before the Rashi tefillin were donned?
The majority of people who put on Rabbeinu Tam tefillin do so either during davening or right after davening, so it stands to reason that the bracha before davening applies for both. There are some people who will put the Rabbeinue Tam tefillin on later in the day. In such a case, the original bracha would not apply to both.
Those who put on Rabbeinu Tam tefillin during davening can learn from tekias shofar that it would not be considered a hefsek. If one made a bracha on the tekiyos and the shofar became disqualified after blowing some kolos, another shofar is then brought to complete the tekiyos. One would not need to make a new bracha when using the second shofar. In addition, we see that the davening of Shemone Esrei is not a hefsek in regard to tekiyas shofar.
One caveat in the case of making a bracha when the shofar is switched would be when a bracha is recited but the shofar becomes disqualified before even one tekiya is blown. When another shofar is brought, one would be required to make a new bracha. The same ruling would apply to the tefillin; if one had not yet completely put on both tefillin of Rabbeinu Tam, then upon switching to the tefillin of Rashi he would need to make a new bracha.
There is a differentiation between the two cases discussed above. When one makes a bracha on a shofar, he only has in mind the shofar that is in front of him and does not think about a shofar that is not there; whereas one who makes a bracha on tefillin does have in mind both his sets of tefillin.
There is a machlokes of our contemporary poskim for a psak halacha and therefore one should consult his own Rav.