Having collaboration with other professors
Professors are people, too. They are not robots to be sent a formal instruction code. Write an email like you are talking to a human. Introduce yourself, introduce your background, clearly explain your proposal. Don't forget to clearly say what is X and Y and why you think working on their combination is a good idea.

Professors are often very busy but they also value good collaborations and will respond if your idea is clearly explained and is promising.