title: Using several Marker Cluster Groups displays overlapping Clusters
body: <p>I am using several <code>L.markerClusterGroup({})</code>'s so that I can switch them in a Layers Control.</p>
  
  <p>But the Clusters hide behind each other.</p>
  
  <p>I would like to be able to get the total number of both Clusters.</p>
  
  <p>What am I missing?</p>
  
  <p><a href="https://i.stack.imgur.com/guXAS.png" rel="noreferrer"><img src="https://i.stack.imgur.com/guXAS.png" alt="overlapping Clusters"></a></p>