a small research. special thanks to @MagicalAsparagus and @henmere for answering my questions and providing some sources (sorry to ping you! i thought you might be interested).

the seven posts from the source:http://armeria.xxxxxxxx.jp/* were uploaded in 2008 by WakuWaku and credited to akita komachi only. in 2013 MagicalAsparagus added horiguchi yukiko in addition to the original artist, and i'll just try to give context why it's very unlikely to be true.

http://armeria.xxxxxxxx.jp/ was indeed a site shared between two artists – akita komachi was drawing and the other was writing – so the site accordingly had two main sections, for drawings and for (BL-)fanfics (based on Tennis No Ouji-Sama only). a banner on a home page looked like this. luckily, among others, we have one of these fanfics archived with a name of the other artist: おまいおこめ (omai okome). the archived home page also shows the URL of the novel section: /okomemenu.html. a profile page has not been archived so we don't know what bits of information could possibly point to horiguchi, but we have the second part, where okome (as komachi usually calls her) tells about her upcoming birthday in the afterword. both posts are Marui Bunta-themed, and above-mentioned link to her section has a news box with recent updates, where the second bunta-related fanfic is listed as being published on 2008/4/17. horiguchi was born in january, and i see no reason to lie about your birthday if you have it posted on your own blog. i'll come back to it later.

apparently, that was also the first place for komachi to post her art, as she refers to okome as the owner of that place. the name was also decided upon around the time of their collaboration, so she took the name of a 'japonica rice cultivar' to match her name ('okome' means [threshed uncooked] rice). furthermore, according to the same news box on okome's page – yet it doesn't seem to be complete – the oldest fanfic was published on 2007/7/21, but the site was rebuilded later to include komachi. 2008/3/20 appears as a founding date on komachi's page and that's probably the reason why okome put her name in the first post after komachi joined – to avoid misunderstanding. in her dairy komachi writes that she messed up with [html] tags and all the data was lost, so she had to create her own site on 2008/7/28. if anything, i couldn't trace where okome moved. komachi often mentioned her and passed the baton to her, which means that she probably had her own place too, but she never linked it anywhere. i have a suspicion that she stopped writing or changed the name.

surprisingly, okome draws too. komachi wrote in a post on 2008/08/25 that okome contributed with two linearts to komachi's one-month anniversary small series of six pictures, each with a Higurashi No Naku Koro Ni character drawing some kind of picture / painting. okome was responsible for the satoko and rena pictures, but none of them have been archived, so if you remember anything similar, please let me know. even when komachi wasn't happy with (her) result, she especially praised the way satoko turned out.

anyway, the likelihood that okome is another pen name of horiguchi is incredibly small (besides, why would she like tennis no ouji-sama that much?). and if we assume that this is true (speaking of birthdays), then there's no need to change your birthday if nothing else points to you, otherwise why would you speculate about a possible connection in your bio? it would be helpful to see those two linearts though, but honestly, i have no clue how MagicalAsparagus came up with her tag, sorry (they don't remember it, if anything). if okome had said that she is dochibi, we would have known it already, otherwise it's just a wild guess. moreover, komachi would have mentioned okome in the same way as she did back then. another wild guess is that 'akita komachi' is a pen name of horiguchi, but other than having some sensibility in common they are particularly different and it doesn't make much sense.