Flopsy said:
+1 to letting two_side_up imply twintails. It makes sense as a subset, and it's just one negated tag to add in those cases where one is only interested in "full" twintails.
I'm not sure there's any meaningful difference between one_side_up and side_ponytail as actually tagged. The wiki for side_ponytail cites flandre_scarlet as an example, but she usually doesn't have all or even most of her hair bunched up in her side tail. That said, +1 to letting the former imply the latter anyway. With that implication, none of the existing side_ponytail taggings will be incorrect, and people can add one_side_up wherever it applies, if they want to bother.
Ah, you're right on that note, save for if there's one character with two side up and another with full twintails, which is what kuudere seemed to imply in forum #123663. It seems to be a "here and there" thing though, but I may be wrong. I haven't looked through multiple_girls two_side_up twintails extensively. Heck, we might even be running into the same problem searching for that already, because again with the assumption of some two side up styles being twintails also.
I do understand the importance of keeping them separate, but the fact of the matter is that a lot of taggers are divided on whether to tag them one, the other, or both in the appearance of a two side up or one side up hairstyle. It's especially apparent since some of the less experienced taggers won't know about two side up and tag twintails anyway because it's what they see.
It's a very unclean divide, and I think that if either twintails or side ponytail are supersets of two side up and one side up respectively, then gardening for both of the latter tags will be much easier than how it is right now.
For now I'll also be +1 on the tentative implications of two side up -> twintails and one side up -> side ponytail, but this is a huge decision; these are tags that are very popular in the database, so before I (or someone else) make the implication I do want others to chime back into this discussion.