Hi there,
you can try moving the secondary nav inside the Primary Navigation like so:
1. Create a new Hook Element:
https://docs.generatepress.com/article/hooks-element-overview/
Give it a title eg. Nest Secondary Navigation inside Primary
Add this to the content text area:
<?php generate_secondary_navigation_position(); ?>
Select: inside_navigation Hook
Check Execute PHP
Set Display Rules tab Location: Entire Site.
2. Customizer > Layout > Secondary Navigation.
Set Container: Full Width, Inner Container: Contained, Alignment: Right, Location: No Navigation
It may require a little CSS to position and probably disable on mobile. Which i can assist with. But this way the two navs are both part of the sticky nav.