Duets are a great way for friends or siblings to share their piano skills together, or parents to get involved with their childrens' interests. It's always encouraging and fun for students to play duets with each other.
As an added bonus, it aids in developing their aural skills in listening for the beat, listening to and playing together with their partner, and can lend to musical discussion on what dynamics they might use in their duo and why, etc.
Another place to find duets of ranging difficulty is here.
And to inspire you, watch this wonderful youtube video below performed by Olga Kharitonova and Igor Machlak (teachers at Melbourne University). Watch their communication throughout - they are perfectly in sync with one another...