One answer

PARSHAS HASHAVUA בא 3 Questions: 1) Why don't the מדקדקים place two Mezuzos on each door post one vertical and one horizontal to be  ? יוצא ידי שניהם ,רש"י ור"ת 2) Why don't we place the פרשיות in the תפילין at an angle to be יוצא ידי שניהם? 3) Why when placing the מזוזה at an angle don't we say that you are not יוצא  according to both Rashi & R.Tam, since it is not vertical and not horizontal? 1 answer: The גמרא מנחות ל"ג ע"א says , עשאה כמין נגר פסולה if you placed the Mezuzah like a bolt/peg it is פסול.   The מחלוקת between Rashi & R.Tam is, what type of  a bolt the גמרא is referring to. (vertical or horizontal)   The מחלוקת is not which position one should place the מזוזה but which position not to place the  מזוזה so that it is not כמין נגר.   All agree as long it is not positioned like a peg it is כשר.   Positioning the מזוזה at an angle is not a compromise (doing it halfway like both) but is the proper לכתחילא   way to place it, (according to both) being a peg is never placed at an angle.   Therefore when placed at an angle there is no need to place two מזוזות. It is positioned according to both Rashi & R.Tam in the proper position.You are יוצא ידי שניהם  by placing it at an angle     This solution won't work by תפילין.    Placing the פרשיות in the תפילין the Halacha dictates how to place the פרשיות and not how not to place it. Therefore by placing it at an angle both Rashi & R. Tam will agree that it would be פסול אמבצי     The גאון who holds to place the מזוזה vertically  (not angle) also holds it's not necessary to put on R.Tam's Teffilin. In both cases since the Halacha is like Rashi there is no need to be יוצא ידי שניהם