js/leaflet.markercluster.js
function (layer, newCluster) {
    if (newCluster === layer) {
      this._featureGroup.addLayer(layer);
    } else if (newCluster._childCount === 2) {
      newCluster._addToMap();

      var markers = newCluster.getAllChildMarkers();
      this._featureGroup.removeLayer(markers[0]);
      this._featureGroup.removeLayer(markers[1]);
    } else {
      newCluster._updateIcon();
    }
  }
function (layer, newCluster) {
    if (newCluster === layer) {
      this._featureGroup.addLayer(layer);
    } else if (newCluster._childCount === 2) {
      newCluster._addToMap();

      var markers = newCluster.getAllChildMarkers();
      this._featureGroup.removeLayer(markers[0]);
      this._featureGroup.removeLayer(markers[1]);
    } else {
      newCluster._updateIcon();
    }
  }
Shared animation code