ג [ב] ומצוה לעשות הריוח שבין פרשת "שמע" (דברים ו,ד) ל"והיה, אם שמוע" (דברים יא,יג), פרשה סתומה; ואם עשה אותה פתוחה, כשרה, לפי שאינה סמוכה לה, מן התורה.The reason why it would be kasher is that in the Torah it is not "סמוכה לה". If this is the reason then why in Chapter 2 is T'fillin not kasher if the p'tuchot and s'tumot are not represented properly? The question is even stronger for the T'fillin Shel Rosh where each parasha is written seperately. Why is it necessary to maintain the form (unlike M'zuza)?
It is interesting to note that in Chapter 5 where the Rambam discusses the Halachot of the tzurot haparshiyot in his enumeration of every and p'tucha and s'tuma in the Torah he does not say what the form of the first parasha (B'reishit, V'Ayleh Sh'mot, etc.) of every Sefer is. I think this is because the first "parasha" is not a parasha at all. A parasha must come after some previous text (this is based on the first two halachot in Chapter 8). At the beginning of a sefer there is no previous text and so it is neither p'tucha nor s'tuma.