The divergent opinions as to the correct order of the Tefillin passages, while seemingly a minor detail, did in fact threaten the unity of the Jewish people. One view was clearly right, one clearly wrong, but which one was which? Although custom has settled on one view, it remains an issue.
The Kabbalah takes a different approach on this question. Each view represents a different reality of Divine perception. Both are needed.
There is a Midrash, that G-d originally planned to create the world through "Midat HaDin" (stern judgment). He saw, however, that Man could not endure this, so he put in "Midat HaRachamim" (mercy/compassion) first, only applying judgment once Man had the proper maturity and perception to deal with this. In the future world, however, the "straight story" will prevail. The will of G-d will be manifest. In the tefillin, the basic debate is whether to put the passage of Shma', which deals with the acceptance of "the Yoke of Heaven"; the love of G-d, before "vhayah im Shamo'a" (this is the view of RASHI) which deals with observance of the Torah, and the concepts of reward and punishment, or whether observance and obedience come first, only later struggling with how this connects us with G-d (this is the view of Rabbenu Tam). In other words, RASHI's view symbolizes how G-d actually made the world, while Rabbenu Tam's view symbolizes how G-d originally INTENDED the world to be.The ambiguity in the Talmud regarding the tefillin was no accident; it allowed for each opinion.The two go hand in hand, representing the acknowledgement of Man's shortcomings, as well as what ideally should be. In addition, RASHIs represent ORDER, as it is the sequence of these passages in the Torah. Rabbenu Tam represent making order out of DISORDER. RASHI's view is the childhood experience; being taught information, being trained step by step on how to live. Rabbenu Tam's view is the adult reality. Nothing goes as we plan.We must learn to "roll with the punches". We must not only survive, but thrive through all the challenges. Still another explanation is that the Rashis represent a female experience (Mochin D'Imma), like a mother who is always there to give order and direction to he children's lives. Rabbenu Tam's are the masculine reality (Mochin d'Abba); struggling in the world, taking risks, in order to provide for his family.The family's order depends on how he deals with disorder! Which is better? They are totally interdependent! Now, not all Jews follow the Kabbalah's injunction to wear both. But among those that do, (Mostly Chassidim and certain Sepharadic communities), some wear both already from Bar Mitzvah. Many others wait to start putting on Rabnenu Tam's until after marriage. The Rabbenut Tam's represent a maturity which might be as yet beyond the comprehensions of an adolescent. They represent an ability (just beginning for the newlywed!) to understand that the genders think and react differently, and happiness can only be achieved when both are appreciated and understood. Some put on both tefillin together, others put on RASHI's first, only after praying putting on Rabbenu Tam's, recognizing that the idea of making order from disorder, of penetrating the role of the male struggle, can only come after the relative calm of the acceptance of G-d, having already prayed, and piercing the veil of the male-female dance in our existence