His assumption is that the Talmud’s statement is a reference to placing two boxes side by side. It can be argued, however, that the Talmud only referred to placing two boxes one on top of the other; going across, however, the space is more limited, as indicated by the Torah’s requirement of “between your eyes”. As R. Shlomo Wahrman (She’erit Yosef, II, 5:1) observes, this is apparently the view of the Radbaz (916) and the Rosh (Hil. Tefilin, 9a, s.v. amar), as well as the Taz (O.C. 32:2), and that both views are noted in the Beit Yosef (O.C. 27).
It should also be noted that the Taz also states that it is unclear if the halakhah actually accepts the view that “there is room on the head to place two tefilin”, and that the Magen Avraham (301:54) writes that in present times we are not sufficiently expert to know where the two boxes would fit. (See also Resp. Hit’or’rut Teshuvah, O.C. 12-13).