R. Daniel Z. Feldman on wearing two pairs of tefillin (link):
The Sanzer Rebbe (Resp. Divrei Chaim, II, O.C. 6) dealt with the question of whether it is appropriate to use a mirror to ascertain that one’s tefilin are properly positioned. (The question is predicated on the assumption that men should not use mirrors, out of concern for “beged ishah”, as stated in Y.D. 156:2; however, see Rama, who notes that in societies in which men also use mirrors, it is permissible to do so). He responded that there is no need to use a mirror for this purpose, in light of the statement of the Talmud (Eiruvin 95b) “there is room on the head to place two tefilin”. If this is the case, that there is twice as much room as is necessary, it would certainly be possible to place one tefilin box within the appropriate parameters without use of a mirror.Once, when I was in Israel for a week, I attended a Sephardic synagogue for all of the services and the rabbi there wore two pairs of tefillin on his head, one in the "normal" place above the hairline and another higher up on the head, which he covered with a big yarmulka. That sight gave me a new understanding of the above-quoted Gemara in Eruvin about wearing two pairs of tefillin at the same time.
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).