unable to select muliple item from listbox because auto postback is true

put the listbox in an updatepanel. That way it will only perform a partial postback. set the update panel to childrenastriggers false and updatemode conditional. Now add a second update panel around your second listbox (same params).

If you never call updatepanel1.update() then the first listbox will not reload and will only post. instead call updatepanel2.update() after you have made the serverside changes

This is a cheap way of implementing an ajax clientside post. There are other ways this can be done but this is by far the simplest