A. One is certainly enough to fulfill the halachic obligation. The 2nd pair is named “Rabbeynu Tam’s” for the 12th century Rabbi who disagreed with Rashi in the order of the Torah portion placed in the head-tefillin. Both agree on the same exact portions and wording, the issue is only about order – and it stems from two possible interpretations of a piece of Talmud. So while the law follows Rashi, it also suggests that those desiring to go beyond the letter of the law might want to wear Rabbeynu Tam’s as well. This is usually done immediately after the morning prayer, for a short time. This practice is mostly followed by Chassidim, especially since there are mystical reasons for the order in Rabbeynu Tam’s Tefillin.