They are turning into one amalgatation
Yeah... if you start having such a strong bond with someone that early in life, you're bound to grow a lot more similar to each other far more readily than if it happened later on. Soon, they will not be Ichikawa and Yamada, they will be YamaIchi. Or something like that.
Kinda like HoriMiya, though they still felt pretty distinct after being an item for about a year (I'm still a bit upset Hori actually cut her hair short after high school and kept it short- I liked her long hair. Same for Miyamura, as a man that keeps his own hair long. Still, it's funny that Souta claims his household has 3 boys instead of 2 boys and a girl- including Miyamura in the count, of course).