Rashi on Deuteronomy 6:8/רש"י על דברים ו׳:ח׳

1וקשרתם AND THOU SHALT BIND THEM [ … UPON THY HAND] — These are the Tephillin that are placed on the arm,

2לטטפות בין עיניך AND THEY SHALL BE FOR FRONTLETS BETWEEN THINE EYES — these are the Tephillin that are placed upon the head. It is in reference to the number of the Scriptural sections contained in them that they are termed טטפת, for טט denotes "two" in Katpi and פת in Afriki denotes "two" (Sanhedrin 4b; cf. Rashi on Exodus 13:16).