Is it possible to create a package within another? I know you can have sibling packages where one of them can reference the other. But what if you already set up your GitHub (or wherever) repo? You can’t have a sibling directory there, unless you rip down everything and start over.
You can’t have two Package.swift in the same root directory, but a package target can specify its source directories, and it can exclude resources including source directories. If you can use those to get distinct file sets for your package targets or packages, it should all work out. (I suppose you can also use directory symlinks.)
cf https://developer.apple.com/documentation/packagedescription/target/sources (which links to resources)