It's basically the same, but you just define the index of the item you want to modify.
Consider the following example
ListBox lb1 = new ListBox();
ListBox lb2 = new ListBox();
lb1.Items.Add("first");
lb1.Items.Add("second");
lb1.Items.Add("third");
lb2.Items.Add(lb1.Items[1]);
lb1.Items.RemoveAt(1);
[ADDED]
Example of moving several items
foreach (int index in new int[] { 2, 0 }) {
lb2.Items.Add(lb1.Items[index]);
lb1.Items.RemoveAt(index);
}
[ADDED]
Since you're using multiselect list as source, you can loop through the selected items. For example
ListBox lb1 = new ListBox();
ListBox lb2 = new ListBox();
lb1.Items.Add("first");
lb1.Items.Add("second");
lb1.Items.Add("third");
lb1.SelectionMode = SelectionMode.MultiSimple;
lb1.SelectedItems.Add(lb1.Items[0]);
lb1.SelectedItems.Add(lb1.Items[2]);
for (int counter = lb1.SelectedItems.Count - 1; counter >= 0; counter-- ) {
lb2.Items.Add(lb1.SelectedItems[counter]);
lb1.Items.Remove(lb1.SelectedItems[counter]);
}