I think this might be using checkboxes? You can select any or all of the values for one attribute option with a checkbox. There would need to be a way of multi-selecting the attributes in the dropdown by pressing the control key while selecting. The Advanced Shipper module has this functionality and inserts all of the selected items from the dropdown to be inserted into the database, I think it's a set of javascript functions. I will check into the files over the next few days and see if I can figure out what needs to be done, at least theoretically, I am not advanced enough to write this functionality for the order editor but can provide code snippets to help out.