Why are there two boxes for the arm Tefillin?

I am not sure what you are referring to, I assume you know what the Tefillin are and you maybe have seen someone placing two Tefillin boxes on his arm simultaneously. There is more than one opinion of how exactly to make the Tefillin. We all follow Rashi's opinion while some have the custom to ware the Tefillin according to his grandson Rabbi Yaakov known as Rabenu Tam as well as Rashi's Tefillin. Most would do it one at a time and some simultaneously.