xxxHOLiC Movie: Manatsu no Yoru no Yume, Tsubasa Chronicle: Tori Kago no Kuni no Himegimi Recommendations
Both are created by CLAMP, plus there's a similar/connected scene that can be seen on both movies. However, I recommend seeing Tsubasa Chronicle movie first as xxxHolic movie carries somewhat of a spoiler for the former than the latter.
They are both by Clamp, and you'll know where the key that Yuko receives goes if you watch the Tsubasa movie..
They're both CLAMP productions that came out on the same day. There's some mystery or twist in both of them.
First of all, everything in the Tsubasa Chronicle world crosses over with xxxHolic, both TV series and movies. In this case, to fully understand the plots of both movies, you should watch them both.