Many GraphQL libraries take some sort of schema definition and then serve it at a route (eg. /graphql). To support multiple schemas, you'd just write a different definition and serve it at a different route. How you resolve the fields is up to you, but both can use shared underlying business logic in these resolvers.
In terms of maintainability, you have to take care that your changes to the underlying business logic don't break assumptions of each schema. And if you want to evolve one schema (eg. say, deprecate a mutation argument, rename a field and deprecate the old naming), you have to ensure that your underlying business logic is backwards compatible for any other schemas (and their clients) relying on it.
In terms of maintainability, you have to take care that your changes to the underlying business logic don't break assumptions of each schema. And if you want to evolve one schema (eg. say, deprecate a mutation argument, rename a field and deprecate the old naming), you have to ensure that your underlying business logic is backwards compatible for any other schemas (and their clients) relying on it.