Continuation Multiple Asynchronous Callouts

To make multiple callouts to a long-running service simultaneously from a Visualforce page, you can add up to three requests to the Continuation instance. An example of when to make simultaneous callouts is when you’re making independent requests to a service, such as getting inventory statistics for two products.When you’re making multiple callouts in the same continuation, the callout requests run in parallel and suspend the Visualforce request. Only after all callout responses are returned does the Visualforce process resume.The following Visualforce and Apex examples show how to make two asynchronous callouts simultaneously by using a single continuation.
public class MultipleContinutationController {
    
    // Unique label for the first request
    public String requestLabel1;
    // Unique label for the second request
    public String requestLabel2;
    // Result of first callout
    public String result1 {get;set;}
    // Result of second callout
    public String result2 {get;set;}
    // Endpoints of long-running service
    private static final String LONG_RUNNING_SERVICE_URL1 = 'http://inqstatsapi.inqubu.com?api_key=27b3c066fcedd91a&data=population&countries=us';
    private static final String LONG_RUNNING_SERVICE_URL2 = 'http://inqstatsapi.inqubu.com?api_key=27b3c066fcedd91a&data=population&countries=gb';
    public List<JSON2Apex> jsonApexURL1 {get;set;}
    public List<JSON2Apex> jsonApexURL2 {get;set;}
    
    // Action method
    public Object startRequestsInParallel() {
        // Create continuation with a timeout
        Continuation con = new Continuation(60);
        // Set callback method
        con.continuationMethod='processAllResponses';
        
        // Create first callout request
        HttpRequest req1 = new HttpRequest();
        req1.setMethod('GET');
        req1.setEndpoint(LONG_RUNNING_SERVICE_URL1);
        
        // Add first callout request to continuation
        this.requestLabel1 = con.addHttpRequest(req1);     
        
        // Create second callout request
        HttpRequest req2 = new HttpRequest();
        req2.setMethod('GET');
        req2.setEndpoint(LONG_RUNNING_SERVICE_URL2);
        
        // Add second callout request to continuation
        this.requestLabel2 = con.addHttpRequest(req2);     
        
        // Return the continuation
        return con;  
    }
    
    // Callback method.
    // Invoked only when responses of all callouts are returned.
    public Object processAllResponses() {   
        // Get the response of the first request
        HttpResponse response1 = Continuation.getResponse(this.requestLabel1);
        this.result1 = response1.getBody();
        jsonApexURL1 = (List<JSON2Apex>) System.JSON.deserialize(response1.getBody(), List<JSON2Apex>.class);
        // Get the response of the second request
        HttpResponse response2 = Continuation.getResponse(this.requestLabel2);
        this.result2 = response2.getBody();
        jsonApexURL2 = (List<JSON2Apex>) System.JSON.deserialize(response2.getBody(), List<JSON2Apex>.class);
        
        // Return null to re-render the original Visualforce page
        return null;
    }
    
    public class JSON2Apex {
        public String countryCode{get;set;}
        public String countryName{get;set;}
        public List<Population> population{get;set;}
    }
    
    public class Population {
        public String year{get;set;}
        public String data{get;set;}
    }
    
    
    public static List<JSON2Apex> parse(String json) {
        return (List<JSON2Apex>) System.JSON.deserialize(json, List<JSON2Apex>.class);
    }
}
<apex:page controller="MultipleContinutationController" showChat="false" showHeader="false">
    <apex:form >
        <!-- Invokes the action method when the user clicks this button. -->
        <apex:commandButton action="{!startRequestsInParallel}" value="Start Request" reRender="panel"/>  
    </apex:form>
    
    <apex:outputPanel id="panel">
        <apex:pageBlock id="result" >
            <apex:pageBlockSection >
                
                Result 1 - {!jsonApexURL1}
                <br/>
                <br/>
                <br/>
                
                Result 2 - {!jsonApexURL2}
                
            </apex:pageBlockSection>        
            
        </apex:pageBlock>
    </apex:outputPanel> 
    
</apex:page>