short checkboxlist

Question

hi


in my webpage have two checkboxlist1 and checkboxlist 2

in checkboxlist1 i click one item then item is remove from checkboxlist and add into checkboxlist2

now again i click in checkboxlist2 the item delete from 2 and add into 1 chekboxlist but

item add in last and not short 

how to short the checkbox list after add any item


please help me......