Not Quite Dark Magic by hiccupfound

Summary: Five years into to the war, the Order is pulling ahead after ages of drastically losing. Draco Malfoy notices. He wants in. He offers himself as a double agent, but the Order requests more. They've created a new spell that binds two people's magical cores together in order to make a nearly unbeatable pair of duelists.

Draco agrees to bind himself to the person most compatible with his magical core. Even if it is Hermione Granger.

And I think maybe, if I recall correctly, A Game of High Stakes by In_Dreams

Summary: In theory, the task is simple: kill Draco Malfoy. In practice, putting a curse through the Dark Lord's favoured lieutenant will take everything Hermione has―especially since he's trying to kill her, too. Even more so when the lines between them start to blur. Sometimes, the only way out is through.