Limiting the number of recognised frame markers to one

Help!! Everyone, im new in vuforia and i now facing a problem when using frame marker, by default frame marker will recognise several frame markers at once, but my project want to do the opposite, i only want the frame marker only recognise and show one result at a time, i already tried set the Max Simultaneous target value to 1 and but the result still the same and i tried to add code below to the defaultTrackableEventHandler script which i found in another discussion and its only work sometime.

Is that any other way to limit the number of simultaneously tracked markers to 1, so that its only recognise one frame marker at a time?

Many thanks in advance. //somewhere

static bool trackedSingleFrameMarker = false;

//then

       public void OnTrackableStateChanged(                                     TrackableBehaviour.Status previousStatus,                                     TrackableBehaviour.Status newStatus)     {                 if (newStatus == TrackableBehaviour.Status.DETECTED ||             newStatus == TrackableBehaviour.Status.TRACKED )         {             if (!trackedSingleFrameMarker )             {                             OnTrackingFound();             trackedSingleFrameMarker = true;         }         }         else         {             trackedSingleFrameMarker = false;             OnTrackingLost();         }     }