In the Wicked fandom, there's Fiyeraba vs. Gelphie. It can get... heated. The Gelphie fandom seems to be the largest, though, probably because of the quite big number of gay people in the musical theatre fandoms.
On fanfiction.net at least this isn't the case. The numbers on either side are pretty even, if anything more for Fiyeraba. Here there's no real combat either, the fanficers simply pretend the other ship doesn't exist - to the extent that if a Fiyeraba writer writes a Gelphie or vice versa they'll be congratulated by their normal fanbase on how original they're being. This troper discovered a load of reviewers she didn't even know existed when she wrote a fic that didn't involve either paring.