Ensure there is sufficient audience overlap and similarities. Nike and Fitbit--yes. Pizza Hut and the American Heart Association--probably not.
Each should have similar objectives and goals; at least one goal should be guaranteed to deliver results.
It offers your targets something unique, like exclusive merchandise, content, or offers.
Create a partnership agreement outlining exactly what each will contribute to the program.
Strive for an equal partnership. Typically, one side will be more well-known or can offer more. While one can deliver more impressions, the other may have deeper data they can tap into.