This is a very poorly coded menu. In order to really fix it you would need to also edit the JS being used.
As a pretty simple work-around, find the following rule in your style.css…
.mobile-menu ul.dropdown-menu.active {
display: block !important;
}
Note: there are a couple instances of .mobile-menu ul.dropdown-menu.active, make sure to edit the one that is not in a media query.
Change the above rule to…
.mobile-menu ul.dropdown-menu.active,
.mobile-menu ul.dropdown-menu {
display: block !important;
}
Now when you click the first menu, the second menu should be open by default.
Since you no longer need the second menu you can just hide it. Add this into your stylesheet…
.mobile-menu .shapely-dropdown {
display: none !important;
}