I'm not getting the same behavior when I view your site and it really shouldn't be happening at all. The grid is designed to wrap both at the same time and in fact is already set to do so.
If it is not happening in a browser/OS combination I'd like to address it as a bug fix so it doesn't happen for anyone.
From your screenshot, which browser and OS is it with? Can you also tell me the version?
Right now the function is in the js>avia,js file:
// improves menu for mobile devices
jQuery('.responsive .main_menu ul:eq(0)').mobileMenu({
switchWidth: 768, //width (in px to switch at)
topOptionText: jQuery('.main_menu').data('selectname'), //first option text
indentString: ' ' //string for indenting nested items
});