I find this (very good) series very similar to http://www.mangaupdates.com/series.html?id=39644

The 2 main heroines are very similar in character to kirino and their relationship is a lot like kirino and kuroneko's, not to mention their looks.
The art style looks very much alike, even though it's not by the same artist.

Anyone else noticed?

That is a pretty common relationship between two female heroines. Even Juvia and Lucy in Fairy Tail have a quasi-similar relationship. I never noticed a similarity in art, but I read what was available Oreimo quite a while ago and part of Boku wa just recently.

Banana Gecko!!
[img]http://www.nodiatis.com/pub/19.jpg[/img]
[img]http://www.wizards.com/magic/images/whatcolor_isblack.jpg[/img][B]

They don't feel the same at all to me. The interaction between the main characters in Tomodachi is more manzai-like (especially between the male character and the rest) and is more similar to D-Frag!. Oreimo, on the other hand, is more drama between the male char and the rest, and more childish fight between the two girls.

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

(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.

Nope, not at all.

Maybe the fact that both names titles "ga <->nai" in it led you to make the connection (half- joking).

"I'll shut your mouth~~~~~ with mine~~~"

二息歩行

I'll see it when Kirino starts trying to shoot Mikuru Beams.