All you have to do is have a javascript function accepting the 2 select boxes as parameters. Loop through the first one and if the current item is selected, add it to the second. Set the current item of the first list box value to null and decrement the current counter after adding the current item's value and text to the second select box at the index of the second one's current length.
something like this...
for(var i=0;i<obj1.length;i++)
{
if(obj1.options.selected)
{
obj2.options[obj2.length]=new Option(obj1.options.text,obj1.options.value);
obj1.options=null;
i--;
}
}