The sub-menu standard behaviour is to align to the left edge of the main menu item. When it gets too close to the edge of the screen it aligns right and a .overflows-viewport class is added to the sub-menu.
When using two menus on the same page, this behaviour is lost. Only the first menu will get the .overflows-viewport class, and the sub-menus on the second menu will always align to the left edge of the main menu item. This will cause the sub-menu to go off screen when it gets too close to the edge and will create some extra horizontal space in the website.