You can enable Above Header/ Below Header section with Header Sections module available with Astra Pro.
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> Layout > Header > Above/ Below Header. Scroll down to the end of the settings and enable the setting Merge menu with primary menu in responsive.
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.