Responsa for Menachot 71:3

אביי ורבא דאמרי תרוייהו לא סח מברך אחת סח מברך שתים

who hast sanctified us by thy commandments and hast given us command concerning the precept of the tefillin!' - Abaye and Raba both said, It means, if he did not speak [between one tefillah and the other] he must only recite one blessing, but if he did speak he must recite the two blessings.<span class="x" onmousemove="('comment',' As reported by R. Hiyya. So that in ordinary circumstances only one blessing is recited, namely at the putting on of the hand-tefillah, which blessing serves for the head-tefillah too. If, however, one interrupted with talk between one tefillah and the other then the second blessing must tk be recited before putting on the head-tefillah. So Rashi and Alfasi, but v. Tosaf. s.v. .');">²</span> One taught: If a man spoke between [the putting on of] one tefillah and the other tefillah, he has committed a transgression and returns home on account of it from the battle line.<span class="x" onmousemove="('comment',' In accordance with the Biblical injunction, 'What man is there that is fearful and fainthearted? let him go and return unto his house (Deut. XX, 8) , which is explained by R. Jose as alluding to the man who is afraid because of his transgressions. V. Sot. 44a.');">³</span>

Teshuvot Maharam