How many brachot are said when donning two pairs of tefillin (Rashi and Rabeinu Tam)? Does it matter how much time has passed between the two donnings (the one before Shacharit and the second after Shmona Esre)?
(I do not refer to taking off a pair of tefillin to use a bathroom and then donning them again. I am asking about wearing two different pairs of tefillin, sequentially or simultaneously, that are donned in order to fulfill the mitzva of tefillin following both Rashi and Rabeinu Tam interpretations of the mitzva.)