Haru to Natsu
Twins, fight over one guy, yandere comedy goodness

I was wondering if there were any mangas like this, doesn't have to be yandere, and I'd prefer school life comedy over extreme action. Something that just has an indecisive guy split between two competitive girls (I know I'm describing the cliche harem here, but it doesn't have to be that expansive or excessive. Just two girls is good).

Mind you, I've read a LOT of mangas, so I'll probably have read most recommondations, but please throw them out here anyways because there is a chance that I haven't read it.

Once again, two girls over one guy, non shoujo please, yandere is good but not a must, just competitive.

I'm pretty sure KissxSis is like this. (haven't read it myself ^^; )

Oniichan no Koto Nanka Zenzen Suki ja Nai n da kara ne!! applies, the rival appears later in the story though.
Girls saurus and its sequel may also fit.
Yomeiro choice too.

I've already read them all, but I'd agree that kissxsis is quite close. As for oniichan no koto nanka zenzen suki ja nai n da kara ne!! i remember reading it awhile back, but if a new rival appears then i'm probably a bit behind. Thanks, i'll check it out.

99% you've read this, but maybe To-LOVE-Ru? Too many harem mangas but that's the 1st I think of.

Domina no Do! perhaps?