I don't think anyone would object to applying the Character Combinations tag to a situation like that, as long as we're still talking Zhu Yilong and Bai Yu (even the real ones). :)

I'm more concerned about situations where a Zhu Yilong or Bai Yu character might be slashed with someone else across fandoms. Like, I don't know, Time Engraver and...anyone who's not played by Bai Yu!