Using Multiple Callouts in a Single Unit Test

TestClassWithMultipleCallouts.cls

@isTest
public class TestClassWithMultipleCallouts {

  @isTest
  public static void testMethodMultipleCallouts(){
  
        String contactResource = 'ContactsMock';
        String invoiceResource = 'InvoicesMock';
  
        StaticResourceCalloutMock contactMock = getStaticResourceCalloutMock(200, contactResource);
        StaticResourceCalloutMock invoiceMock = getStaticResourceCalloutMock(200, invoiceResource);
        
        Map<String, HttpCalloutMock> mocksMap = new Map<String, HttpCalloutMock>();
        
        mocksMap.put('https://api.xero.com/api.xro/2.0/Contacts', contactMock);
        mocksMap.put('https://api.xero.com/api.xro/2.0/Invoices', invoiceMock);

        HttpCalloutMock mocks = new MultiRequestMock(mocksMap);
        
        Test.setMock(HttpCalloutMock.class, mocks);
  
    		// act
        Test.startTest();
        // make call out
        Test.stopTest();
    
        // assert
        
  }
  
  private static StaticResourceCalloutMock getStaticResourceCalloutMock(Integer statusCode, String staticResourceName) {
        
        StaticResourceCalloutMock mock = new StaticResourceCalloutMock();
        
        mock.setStaticResource(staticResourceName);
        mock.setStatusCode(statusCode);
        mock.setHeader('Content-Type', 'application/json');
        
        return mock;
  }
            
  public class MultiRequestMock implements HttpCalloutMock {
        
        Map<String, HttpCalloutMock> requests;
        
        public MultiRequestMock(Map<String, HttpCalloutMock> requests) {
            this.requests = requests;
        }
        
        public HTTPResponse respond(HTTPRequest req) {
            
            HttpCalloutMock mock = requests.get(req.getEndpoint());
            
            if (mock != null) {
                return mock.respond(req);
            } 
            return null;
        }
        
        public void addRequestMock(String url, HttpCalloutMock mock) {
            requests.put(url, mock);
        }
    }          

}