xxxHolic (Or the Lack of the Manga)
Apparently I lost the copy of xxxHolic I have... that is if I even had it to begin with. It's weird because I recall buying the manga and it wasn't cheap. (Then again, all Del-Rey manga isn't really cheap at all.) However, I do have Tsubasa: Reservoir Chronicles volume one and it mentions the fact that the two volumes interact. Hell, there's even a small part where the charactrs from Tsubasa interact with Yuko and Watanuki. Only reason why I even attempted to hunt down the xxxHolic manga is because of a scene where Maru and Moro uses Fai's staff as a club to dust a futon.