How to add multiple item in a list box from another listbox which have SelectionMode =Multiple

Question

I have two listbox lst1 and  lst2, lst1 have 'SelectionMode =Multiple' ,Please tell me how to add multiple  item in lst2 when we select multiple item in lst1.

For example 

In lst1 if we select two states UP and AP then we want all cities of AP and UP added in lst2 .

Thanks in advance