Listbox Sorting

I have 2 listboxes, where the user can add and remove from listbox to listbox which works fine, but everytime you remove an item from the 2nd listbox and add to the 1st, I would want the 1st listbox to sort.  How do I do this this?  If i do Listbox.DataBind() it includes items which are present in the 2nd Listbox.  The 1st Listbox has a SQLDataSource bound to the control.

Cheers,

Jon