Incorporate location selection drop-down widget into split panel Earth Engine App

You were almost there. Inside the onChange function in ui.Select object, you were setting the center of a new ui.Map object whereas what is being displayed on screen are leftMap and rightMap objects.

So change the function to:

var select = ui.Select({
  items: names.getInfo(),
  placeholder: ('Choose Area of Interest'),
  style: {width: '290px'},
  onChange: function(key) {
    // Map.clear(); //clears previous calculations from map upon new selection
    var selection = ee.Feature(fc.filter(ee.Filter.eq('name', key)).first());
    // print(selection)
    leftMap.centerObject(selection); //centers map on Plantation selection
    // show Plantation
    var layer = ui.Map.Layer(selection, {color:'blue'}, 'Plantation');
    leftMap.layers().set(1, layer); //adds Plantation selection to the map
    // rightMap.layers().set(0, layer); //adds Plantation selection to the map
    var conversion = ee.FeatureCollection(selection); //converts the Plantation selection into a feature collection for future analyses
  }
});

And the drop-down location selector will work as intended.

Link to modified code.