Identify use cases where both products come together to compliment one another or build a better solution together. Another way to approach it is by identifying products that share similar/same personas.
As an example, we have a Marketing product and a CMS product. Those two products go together well, because oftentimes marketers are not only in charge of their campaigns, but also the website. We also know that for a marketer to build a proper digital experience / buying experience for their customers, they need to consider all the touchpoints across email, ads, social, AND the website. Bring those together.
Always keep the customer in mind. Ask yourself "why would a customer need both products?" and "how would these two products compliment each other and provide the customer more value than if it was a stand alone?" -- the answers to those will likely lead to some ideas on multi-hub content.