Sometimes the simplest things elude us. One feature not commonly understood in LTspice is how to copy and paste between schematics using the duplicate command. To copy objects from one schematic to another, in the source schematic, invoke the duplicate command (F6 or Ctrl + C)—the crosshair pointer changes to the duplicate symbol. Left-click to select the object you want to duplicate, or select a group of objects by dragging a box around them.
Once the object or section is copied, simply click in the target schematic window, navigate to the location you want to paste and click again. In Windows, both schematics must be in the same invocation of LTspice.