That doesn't seem accurate. For starters, multiple authors worked under one name isn't that unusual (e.g. Mochi Au Lait). Then there's the fact that both members of Nico Tanigawa (Watamote's two authors) worked on all three of their series together: Watamote, Number Girl, and Fake Literature Girl.
According to interviews of the authors, Ikko, the male author, writes and Niko, the female author, draws; with some overlap between the two's roles. From what I've gleaned, Ikko is responsible for most of the story and characterization, and consults Niko on how to write believable high school girls.
I suspect Niko is a raging lesbian, either closeted or latent, and her homosexuality has been gradually seeping into Tomoko's characterization since the beginning of the manga. Either discussing it with Niko directly, or picking up on it on his own, Ikko eventually began working the "Tomoko is into girls" angle into the story explicitly. Their audience responded well to it, so they've been rolling with it ever since.
That of course, is all speculation, made through a language barrier, at that.