When general mills teams up with hershey foods to create a cereal called reese's peanut butter puffs (the reese's brand belonging to hershey's), what is the general mills' branding strategy called?

It is known as the Co-marking Strategy. A partnership among no less than two distinct brands of products or administrations. Cobranding envelops a few distinct sorts of marking organizations, for example, sponsorships. This methodology normally relates the brands of no less than two organizations with a particular decent or administration.