Hi @artthescience,
Thanks for writing in!
As you are using Pro header builder, you can make it easier.
Please follow the steps.
1. Create another menu (Appearance -> Menus) with all the items from both the menu. Name it mobile menu.
Edit the header in header builder and hide both the menu left and right in the mobile device.

2. Then you can add any mobile navigation like navigation modal, Navigation layered as per your choice.

Then chose the mobile menu there.

3. Heide the menu elemnt in larger screen.

Hope this helps!
Thanks