The mishnah states that one who writes 2 identical letters is chayuv (according to rashi's girsah), and Rashi explains this to mean two alephs, even though they are not read together to form a word. According to Rashi, why is there a requirement that the two letters must be next to each other (104b), if their chashivus is not in their being read together? Maybe it's because in the mishkan they wrote two identical letters to match up the kerashim, so their chashivus was in being read together (Avnei Neizer).
Thank you, Eliezer Levine
Thank you for supplying both an excellent question and an excellent answer. The only thing I have to add is that Rabeinu Perachyah (Shitas ha'Kadmonim on Shabbos 103a) also says (in the Tana Kama on 103a) that they used to write the same exact letter on the two Kerashim next to each other in order to match them up.
All the best,
Yaakov Montrose