Co-branding is a partnership where two distinct brands collaborate on a joint product, campaign or piece of content, typically presenting both logos together to signal the association to customers.
The design challenge is giving both brands fair, balanced visual weight without either identity clashing with or overpowering the other — usually resolved through careful spacing, a neutral dividing element, and consistent sizing between the two logos.
Most brand style guides don't anticipate co-branding scenarios by default, so it's worth having a designer establish simple rules (spacing, relative sizing, acceptable layouts) for how your logo should appear alongside a partner's when the situation comes up.