In that case you would not use the Secondary Navigation.
Instead enable the Off Canvas panel in Customizer > Layout > Off Canavs. Set this to display mobile and desktop.
Create 2 x menus:
1. Categories – set location to the Primary Navigation.
2. Other menu items – set location to Off Canvas Panel
This will give you the two menus on desktop and the single menu on mobile.
Once thats setup – share a link to your site and we can take a look at creating that layout.