Ahoj Petra,
by default is NOT possible, it request using Custom Header.
You can check php code that we used in eShort header php file - but I'm afraid this is not a simple task, and beyond support.
As an alternative, I can recommend another solution. But it will be with simplified features(!)
- Disable predefinded header
- Using Helix Ultimate documenation create a custom header (Template Options > Layout)
with 3 columns, and structure: 4+4+4 (menu, logo, top3) , guide: https://www.joomshaper.com/documentation/helix-framework/custom-header
- Then on position
top3 add & publish 2nd menu - use MENU module and add styling to make it horizontal. This class name should help: Advanced (tab) > Menu Class (field): navbar
This should work, but MegaMenu would be only on the left side (!).