Above Header/ Below Header section with Header Sections module is a premium feature available with Astra Pro Addon plugin. To use these Pro features, you need to have the Astra theme along with the Astra Pro Addon installed on your website.
If you have set menu with either section in above/below header, you will see a separate hamburger for it in responsive devices.
If you are displaying a primary menu and above/below header menu- both, then you will see two separate hamburger menus in responsive.
If you do not wish to show two separate hamburger menu, you can merge them with a simple setting.
From dashboard navigate to Appearance > Customize > Header > Above/ Below Header. Scroll down to the end of the settings and enable the setting Merge Menu on Mobile Devices.
It will merge the header section’s menu with a primary menu in responsive. Primary menu items will be displayed on top of the header section’s menu items.