DropDownList change the values in the same times

Question

I have got two DropDownLists, and I want to change if I change in first Drop down list Values then I want to change in secound DropdownList. My first DropDownList has got a code:

 SPUserCollection userzyWszyscy2 = witryna.SiteUsers;

        // Listowanie userów alfabetyczne

        var listUsers = from SPUser u in userzyWszyscy2
                        orderby u.LoginName
                        select new ListItem(u.LoginName.ToLower());

        DropDownList1.Items.AddRange(listUsers.ToArray());

And secound:

  SPUserCollection userzyWszyscy2 = witryna.SiteUsers;

                // Listowanie userów alfabetyczne

                var listUsers2 = from SPUser u in userzyWszyscy
                                 orderby u.LoginName
                                 select new ListItem(u.LoginName.ToLower());

                DropDownList3.Items.AddRange(listUsers2.ToArray());

How Could I corelate them toogether?

If I change in One I will change in Two.