Combo setSelectionMode multiple

Does anyone know how to do a multiple-select Combo box? I have tried the following in 2.0/2.0.1 on IE and Firefox, but it does not seem to work...

var obj = new AW.UI.Combo;
obj.setControlText("Combo");
obj.setControlImage("favorites");
obj.setItemText(["Home", "Favorites", "Font size", "Search"]);
obj.setItemImage(["home", "favorites", "fontsize", "search"]);
obj.setItemCount(4);
obj.setSelectionMode("multi"); // my best guess...
document.write(obj);

Thanks,
Andrew.