Do you have an example of where you have seen this so we can dissect it for you?
I can think of various ways of making two lists into one such as setting 50% width on the list elements so that they are floated side by side and then at small screen widths make them 100% so they fall vertically. Or you could use two separate uls with both blocks floated but at 50% width on larger screens and then 100% width and not floated on smaller screens so that the second block lines up underneath.
Or (although a little wasteful) you could have two unique menus with different structures and hide one or the other at the various screen width as required by the media query. A lot of mobile sites do this and display a menu button instead of a whole menu.