These sorts of changes are quite far into the custom development territory so not really part of normal theme support. We’ll do our best to advise.
A friendly reminder to work from within a child theme to keep your changes update safe. All WordPress.org themes completely overwrite their own folders during the update process. If you make changes to the parent theme and run a theme update, all changes will be lost.
Once you’ve registered your menu in the child functions.php file, make a copy of header.php and insert it in the child theme folder.
Next go to /inc/template-tags.php and locate line 129. Copy that entire function into your child theme functions.php file. The function starts with:
and ends with:
The entire function must be copied into the child functions.php file. You can then add your second menu.
There will most likely be extra CSS adjustments required. If you can resolve the above then you’d be most of the way there.