If you own two modules for the same game system, and want to run a campaign that uses a blended mix of materials, there should be a way to either merge modules, or use a second module as an addon. As an example: I would like to run a campaign in DnD5e Lost Mines of Phandelver and then sprinkle in monsters, maps, and other content from Dragon of Icespire Peak (doubly so since they both take place in the same region and feature the same starter town)