Further to your real code, here is how I would do it. I haven't tested the following code though.

 #region Checking For Internet
 public void CheckForInternet( System.Action onInternetAvailable, System.Action onInternetUnavailable )
 {
     if( !IsInternetReachable() )
     {
         if( onInternetUnavailable != null )
             onInternetUnavailable();
         return ;
     }
     
     StartCoroutine( CheckForInternetConnectivityIEnum( onInternetAvailable, onInternetUnavailable ) ) ;
 }
 
 private bool IsInternetReachable()
 {
     return Application.internetReachability == NetworkReachability.ReachableViaCarrierDataNetwork || Application.internetReachability == NetworkReachability.ReachableViaLocalAreaNetwork ;
 }
 
 System.Collections.IEnumerator CheckForInternetConnectivityIEnum(System.Action onInternetAvailable, System.Action onInternetUnavailable)
 {
     WWW www = new WWW ("some api w$$anonymous$$ch gives sucess 1");
     yield return www;
     if ( www.error == null )
     {
         JsonData jsonDat = JsonMapper.ToObject (www.text);
         Debug.Log( "Login API success = " + jsonDat["success"] );
         if ( jsonDat["success"].ToString() == "1" )
         {
             if( onInternetAvailable != null )
                 onInternetAvailable();
         }
         else
         {
             if( onInternetUnavailable != null )
                 onInternetUnavailable();
         }
     }
     else if( onInternetUnavailable != null )
         onInternetUnavailable();
 }
 #endregion

And call it as follow :

 public void Foo()
 {
     CheckForInternet( OnInternetAvailable, OnInternetUnavailable ) ;
 }
 public void OnInternetAvailable()
 {
     Debug.Log("Internet is available");
 }
 public void OnInternetUnavailable ()
 {
     Debug.LogWarning("Internet is unavailable");
 }

INITIAL ANSWER

I don't t$$anonymous$$nk what you are trying to ac$$anonymous$$eve is possible without more coroutines and callbacks. Here is my suggestion:

     bool one = false;
     bool two = false;
     public void A( System.Action<bool> onResultFetched )
     {
         StartCoroutine( B( onResultFetched ) );
     }
     private System.Collections.IEnumerator B( System.Action<bool> onResultFetched )
     {
         one = false;
         yield return StartCoroutine( C() );
         onResultFetched.Invoke( one );
     }
     private System.Collections.IEnumerator C()
     {
         two = false;
         yield return new WaitForSeconds( 1f );
         if ( somet$$anonymous$$ng is true )
             one = true;
         else
             one = false;
         two = true;
     }

And then, you call A with a callback:

     private void Test()
     {
         A( ( result ) => Debug.Log( result ) );
     }