How is the correct way to have multiple dataProviders in react-admin?

You're not calling the dataProvider you found, you're calling the mapping object from your array. You can fix it like this:

import simpleRestProvider from 'ra-data-simple-rest';

const dataProviders = [
  { dataProvider: simpleRestProvider('http://path.to.foo.api1'), resources: ['users1'] },
  { dataProvider: simpleRestProvider('http://path.to.foo.api2'), resources: ['users2'] },
];

export default (type, resource, params) => {
  const dataProviderMapping = dataProviders.find(dp => dp.resources.includes(resource));
  return dataProviderMapping.dataProvider(type, resource, params);
};

If you're wondering how to have multiple data sources as well as multiple dataProvider types i.e. rest, GraphQL e.t.c. The v3 docs state:

In react-admin v2, Data Providers used to be functions, not objects. React-admin v3 can detect a legacy Data Provider and wrap an object around it. So Data Providers developed for react-admin v2 still work with react-admin v3.

This means that the dataProvider property on the <Admin> component can take:

• An object of type DataProvider
• A function that provides (type, resource, params) returning a promise containing the data.

This means that if you provide your DataProvider is a function, you will have to create an adapter and call the functions on your provider manually. Here is an example.

./dataProviders.js

import { gqlDataProvider, restGQLDataProvider, restProvider } from './buildDataProviders';

// The first assumption is that all resources are unique, thus we can use an object or a map instead of an array. Key = resource, value = DataProvider.
const dataProviders = new Map([
  ['users', restGQLDataProvider],
  ['users-local', restProvider],
  ['Invoice', gqlDataProvider],
  ['Debtor',  gqlDataProvider],
  ['Status',  gqlDataProvider]
]);

export default async (type, resource, params) => {
  // Get the DataProvider from the map.
  const dataProvider = await dataProviders.get(resource);

  // The DataProvider object is wrapped in a function so we can pass in the parameters to get the result.
  if (dataProvider instanceof Function) {
    return dataProvider(type, resource, params);
  }
};

./buildDataProvider.js

import { gql } from 'apollo-boost';
import buildGraphQLProvider from 'ra-data-graphql-simple';
import { gqlClient, restClient } from './apolloClients';
import simpleRestProvider from 'ra-data-simple-rest';

const LOCAL_REST_ENDPOINT = 'http://localhost:3001/api';

export RestProvider = simpleRestProvider(LOCAL_REST_ENDPOINT);

export const gqlDataProvider = buildGraphQLProvider({
  client: gqlClient
});

// This is just an example of the getList provider, you'll have to write the rest.
export const restProvider = (type, resource, params) => {
  const providerMap = new Map([
    ['GET_LIST', async () => { 
      return await RestProvider.getList(resource, params);
    }]
   ])

export const restGQLDataProvider = (type, resource, params) => {
  const providerMap = new Map([
    ['GET_LIST', async () => {
      const query = gql`
        query ${resource} {
          ${resource} @rest(type: "${resource}", path: "/${resource}") {
              id
              name
              username
              email
              address
              phone
              website
              company
            }
          }
        `;
      
      const result = await restClient.query({ query });
          
      return { data: result.data[resource], total: result.data[resource].length };
    }]
  ])
}

./apolloClients.js

import { ApolloClient, ApolloLink, InMemoryCache, HttpLink } from 'apollo-boost';
import { RestLink } from 'apollo-link-rest';
import auth from '../../auth';

const GRAPHQL_ENDPOINT = 'http://localhost:4000';
const REST_ENDPOINT = 'http://jsonplaceholder.typicode.com';
const httpLink = new HttpLink({ uri: GRAPHQL_ENDPOINT });
const authLink = new ApolloLink((operation, forward) => {
  operation.setContext({
    headers: {
      'x-api-key': auth.aws_appsync_apiKey
    }
  });

  return forward(operation);
});

export const restClient = new ApolloClient({
  link: new RestLink({ uri: REST_ENDPOINT }),
  cache: new InMemoryCache()
});

export const gqlClient = new ApolloClient({
  link: authLink.concat(httpLink),
  cache: new InMemoryCache()
});