|Yeah, as swichgear says, it's pretty common.
And I agree with Damnedman; the dynamics are completely different in how they act toward the main character, each other, and how the story is presented in general. Kirino and Kuroneko trade blows at somewhat equal intervals, and no one really has the upper hand. Yozora and Sena confrontations are usually a swift curb-stomp battle in Yozora's favor. Even when Sena manages to score a hit, Yozora takes it, brushes it off, and yadda yadda yadda you'll see Sena running out the room crying in a few pages.
I'd say the one big similarity is that both Kirino and Sena inadvertently insult their counterpart, and have no clue they're doing it.
Spoiler level: Mild
Spoiler (mouse over to view)
(IE Much to Yozora's bane, she simply can't beat Sena in sheer academics no matter how hard she tries, but Sena's doesn't seem to exploit it. From what I remember of Oreimo, Kirino never realizes how much her novel being published burns Kuroneko's ass.)
The art styles are way different though. I don't see a similarity there at all.