Said Rabbah: Rav Huna explained it to me. The verse states “and they shall be as tefillin between your eyes”—they, in the plural—to imply that at any time that there is tefillin between your eyes, there shall be both (i.e. both the head- and the arm-tefillin).
(Talmud, Menachot 36a)
We read in Shmoth 33.6 “And the children of Israel stripped themselves of their ornaments from mount Horeb onward.” And in Shabbat 57b: “Rather, Rav Yehuda said in the name of Abaye: A totefet is an appuzainu, an ornament worn on the forehead.”
When we say a Bracha over the Hand tefillin we say it with the intention that the Bracha covers the Head tefillin as well. It is explained that the single bracha covers both because, it says, “bind them for a sign upon your arm and let them be for a remembrance between your eyes;” and them is written in the plural –
See also Ibn Ezra to Shmoth 13.9 citing Koheleth (Proverbs) 1.9, 3.3, 6.9
How can the Shel Rosh (head tefillin) serve as a sign if we do not discuss the Torah when we see it? The passuk says, “and as a reminder on your forehead—in order that the Teaching of the L-RD may be in your mouth” Shmoth 13.9
Obviously, if seeing the Shel Rosh initiates discussion of the Exodus, (as Ibn Ezra elaborates) then the head tefillin serves as a sign right along with the hand (arm) tefillin – The Torah stipulates that they are to be worn together, “in order that the Teaching of the L-RD may be in your mouth” Shmoth 13.9
I understand the mitzvah as: “and it shall be for a reminder between your eyes [and my eyes,”] that is, when each person sees the totafoth (shel rosh) on the head of a fellow Jew it shall serve as a reminder to discuss the Exodus; for “with a strong Hand, HaShem took you out of Mitzrayim.”
Shmoth 13.8 stipulates that during the Pesach season we are to discuss the Exodus: “And thou shalt tell thy son in that day, saying: It is because of that which the L-RD did for me when I came forth out of Egypt.”
Obviously, Shmoth 13.9 is teaching us that seeing the Shel Rosh serves to initiate discussion of Pesach and the Exodus; something the Shel Yad cannot do once it is placed opposite the heart!