פירוש על מנחות 71:18

Between the placing of one of the Tefillin and the other one - Between that of the hand and that of the head. He spoke before he placed on the one of the head.

To lay on - Because he starts laying with that of the hand.

The precept of Tefillin - Because now he finishes the precept.

If he did not talk - He only blesses on that of the hand alone.

If he did not talk he says one blessing, if he talked he says two - Rashi explains: If he did not talk he says one blessing - for both that of the hand and that of the head. If he talked, he says two blessings - "To lay the Tefillin" for that of the hand, and "concerning the precept of Tefillin" for that of the head. But Rabbeinu Tam explained: If he did not talk, he says one blessing - On that of the head he says (only one blessing) for that of the head: "Concerning the precept of tefillin," because that is its blessing. If he spoke he says two blessings - for that of the head: "to lay" and "concerning the precept of." And it is also written like this in the Shimusha Rabba D'Tfillin. This is the teaching of my teacher and master.

If he talked he says two blessings - Like Rav Chiya sent.