Overlaying modes is a feature in the regular geometry display.
When you drag and drop a mode in the display, you will notice that modes dropped on the left highlight about 3/4 of the screen. When you drop in this area, it will display the mode.
When dropping the mode on the right 1/4 area, this will create an overlay.
The phasing between the two overlaid modes can be adjusted using the "Animation Control" (right click in display). This allows you to phase align them as desired.
Hope that helps!
The Left / Right and Upper / Lower geometry displays can show two mode shapes besides each other or above eachother. To overlay them, use the standard Geometry display and drag one mode on the Left (Main) area and one mode on the right (Overlaid) area.
To remove one, right click in the display and select Remove Animation, Main or Overlaid.