How To - Photoshop Script - Merge Two Layers

Presuming that the upper Layer X Copy layer is active/targeted, the following line of code would merge Layer X Copy into the lower Layer X directly underneath in the layer stack:

app.activeDocument.activeLayer.merge();

(The equivalent of the Layer > Merge Down or CMD/CTRL + E command)

P.S. I tried your code, however, I am guessing that something was lost in the copy/paste or that the formatting has messed things up too much.

Perhaps something like this?

(function ()
{ var docRef = activeDocument 
    var newdLayer = docRef.activeLayer.duplicate(); newdLayer;    
    var startRulerUnits = app.preferences.rulerUnits;    
    app.preferences.rulerUnits = Units.PIXELS;     
    var bounds = newdLayer.bounds;     
    var height = bounds[3].value - bounds[1].value;    
    var newSize = (100 / height) * 600;    
    newdLayer.resize(newSize, newSize, AnchorPosition.MIDDLECENTER);   
    app.preferences.rulerUnits = startRulerUnits;  
    var blur = docRef.activeLayer;    
    var startRulerUnits = app.preferences.rulerUnits;    
    app.preferences.rulerUnits = Units.PIXELS;     
    var bounds = blur.bounds;     
    var width = bounds[2].value - bounds[0].value;    
    var newSize = (100 / width) * 900;    
    blur.resize(newSize, newSize, AnchorPosition.MIDDLECENTER);   
    app.preferences.rulerUnits = startRulerUnits;    
    blur.applyGaussianBlur(5); 
    }
)
();
//////////////////////////
select(false);
function select(makeVisible) {
  var c2t = function (s) {
  return app.charIDToTypeID(s);
  };
  var s2t = function (s) {
  return app.stringIDToTypeID(s);
  };
  var descriptor = new ActionDescriptor();
  var list = new ActionList();
  var reference = new ActionReference();
  reference.putEnumerated( s2t( "layer" ), s2t( "ordinal" ), s2t( "forwardEnum" ));
  descriptor.putReference( c2t( "null" ), reference );
  descriptor.putBoolean( s2t( "makeVisible" ), makeVisible );
  list.putInteger( 13 );
  descriptor.putList( s2t( "layerID" ), list );
  executeAction( s2t( "select" ), descriptor, DialogModes.NO );
}
//////////////////////////
app.activeDocument.activeLayer.merge();