Tsubasa Chronicle- Princess Sakura activates some old ruins and loses her memories which are spread out into many different parallel worlds in the form of feathers. Sakura is dying and in order to save her Shouran makes a deal with Youko a dimensional witch. However, in order to gain the power to travel to alternate worlds and retrieve her memories he must pay a price in exchange. Sakura will no longer remember him, all the memories they shared growing up as well as their love for each other is erased as Shouran's penance. The dimensional witch gives him a way to communicate with her and travel dimensions as well as two companions who arrived each seeking other things.
XXXHolic-Watanuki is a boy plagued and tormented by spirits attracted to him like a magnet, that just won't leave him alone. One day he stumbles across the home of Youko the dimensional witch, and she promises to grant his wish to be free of the spirits. In exchange he must work for her, and become her servant/lackey.
Both of them are by the same people and made simultaneously. They also frequently cross over. I can't say anything about the mangas as don't read those two, but their anime incarnations aren't bad at all, despite being a little repetitive.