Stone and Starfire

A Cardcaptor Sakura and Magic Knight Rayearth Crossover by Jade Wing, the story takes place after both series have concluded. The Knights discover a text about two over-powered Knights, who are then summoned. Turns out, they already know how to use magic-but their magic doesn't work in Cephiro. Those two knights are Syaoran and Sakura and they've been separated for a while -- for two years, in fact.

Once the heroes arrive in Cephiro, things go as well as can be expected until Sakura is captured, and her voice and knowledge of writing get sealed away. Things only get better from there.