User blog comment:Master Sima Yi/Assassinews 05-08-'14 – Assassin's Creed: Rogue announced/@comment-25344053-20141106180850/@comment-3430127-20141106214504

In regards to the two hidden blades thing, that was more or less started by Altair because the "metals with which we forge them remain difficult to obtain" (Quote from his codex pages), after that it simply became a requirement that the Assassin was deemed skilled enough to use them. Besides, I'm pretty sure Achilles didn't really care about whether he gave the only Assassin in the area two blades. Also in Black Flag, Julien du Casse said two was the tradition, which suggests in the 200 years after Ezio became an assassin, they figured two blades were more practical. (Damnit, Everytime I'm about to finish I remember something else) Speaking of Ezio, he got his second blade in 1478, two years after his father and brothers died, and he wasn't actually a formal Assassin until 1488, but nobody, ingame or here, has ever complained that he had two blades.