Conceptually. He has half the traits of Ryougi and half the traits of Mikiya; that is, he was created as more or less the combination of the two. And he was created afterwards.
He's a bit like them, I agree with that. Still, I'm pretty sure Tsukihime came first. Even if it didn't, Nasu never said he created Shiki based on the concepts of Ryougi and Mikiya, so you're just making things up based on your own impression of this.
Actually, Kara no Kyoukai one of his earliest works, releasing the light novels from '98 - '99 (Tsukihime was released in December 2000). I'm pretty sure Nasu has said it somewhere or the other but I'm not going to look through all his interviews for that; it's fairly obvious, don't you think? Personality- and looks-wise, Tohno Shiki and Mikiya Kokuto are extremely similar, all the way down to being "ladies' men". Then Tohno has the Mystic Eyes, and his other personality resembles Ryougi.