Switch one community to another community based on single user login

Is this possible to switch the community portal using single login?

global CommunitiesLoginController(){
        returnURL = ApexPages.currentPage().getParameters().get('returnURL');
        if(returnURL <> null){
            startURL = '/'+returnURL.substring((returnURL.lastIndexOf('/')+1));
        }
        System.debug('RETURN URL>>>>>>>.'+returnURL);
        sYSTEM.debug('startURL>>>>>>>>>'+startURL);
    }
    global PageReference login() {
        System.debug('Username>>>>>>>>>>'+login_Username);
        System.debug('Password>>>>>>>>>>'+login_Password);
        //Check Whether is already existing or Not
        if(returnURL <> null){
            PageReference authorized = Site.login(login_Username,login_Password,startURL);
            system.debug('authorized>>>>>>>>>');
            if(authorized <> null){
                List<User> userList = [Select Id,ContactId FROM User Where Username=:login_Username];
                System.debug('Result user>>>>>>.'+USERLIST.SIZE()+'Contact Id>>>>>>>'+userList[0].ContactId+'User Id>>>>>>'+userlist[0].id);
                if(userList.size() > 0){
                    List<Navigator_Contact__c> nc = [Select Account_Info__r.Order_Access__c FROM Navigator_Contact__c Where Parent_Contact__c=:UserList[0].ContactId];
                    sYSTEM.DEBUG('nAVIGATOR RESULT>>>>>'+nc);
                    if(nc.size() > 0){
                        if(nc[0].Account_Info__r.Order_Access__c == true){
                            system.debug('I am User but I have Order Navigator Access'+returnURL);
                            return authorized;
                        }
                    }
                }
            }