Twenty Twelve collapse sidebar menu to a button

I’m developing a site with a child of the twenty twelve theme. If you look at this page my web-site you will see there is a horizontal navigation menu and a sidebar submenu down the left side. When the window is reduced in size or the page is viewed on a smart phone the horizontal main menu collapses (which is good) to a button but the sidebar submenu doesn’t. How can I collapse this sidebar submenu to a button that will expand when clicked, with the main menu button remaining there? Ideally the sidebar button would be alongside the main menu button and under it when the main menu is expanded – but this is not too important.