Hi Ross,
Thank you for contacting Styled Themes.
If we follow standard way of building sub-menu then first top most menu should be kept with # link, then you can keep any pages/post as sub-menu.You can follow as per our demo: https://demo.styledthemes.com/demos/puresimple/. Please find the attached file for menu setup on demo.
if you have added link on top menu then that top menu will open on double click, on single click it will open sub-menu.
Let us know if we are understanding you fully or perhaps this is not what you meant for.
We look forward for your response.
Thanks