Schrobby said in forum #92931 this:
Twintails only apply when the bulk of the hair is used, everything else is two_side_up. Following this definition long_hair combined with short_twintails is simply not possible, so that part should be deleted from the wiki.
I don't think this is true though. Maybe at the very least not anymore. Nowhere yet does it say that the bulk of the hair has to be tied up into two twintails in the twintails wikipage. And as I mentioned in forum #123629:
The "tails" of a two side up hairstyle can be anywhere between relatively short to as long as the hair drooping down.
I have never seen exceptions to the case though -- there most likely aren't any. If in the case where you can't see the other tail of a two side up hairstyle it could linger between either a two side up or a side ponytail -- you would probably have to rely on your knowledge of the character to decide what fits best, or skip tagging the hairstyle altogether.
EDIT: Ah, right. A potential mixup between the two could be for example, viewing shimada arisu from the side. She has a side ponytail, but it is deceivingly styled like one side of a two side up hairstyle.