This option is only available if the Multi-Plot window is active. If you don't have a Multi-Plot window, you can open it with Window - New Window - Multi-Plot.
This menu creates a new sub-view (Plan, Profile, Section, Data, Rectangle etc.). You can have multiple occurrences of each sub-view type.
NOTE: the new sub-view is created in the center of the Multi-Plot sheet; if you are zoomed in, you may have to End Zoom to see the new window.