I have recently seen a Chasid wearing two baatim (Rashi and Rabeeinu Tam) on his head but only (Rashi) for Tefillin Shel Yad. Later, a third Tefillin Shel Rosh were applied but the Shel Yad stayed put. Is this halachically correct?
Some have the custom of wearing both at the same time. The ou site has a discussion concerning Rashi and Rabbeinu Tam tefillin. See point 4.