I have a double gang switch at bottom of stairs. It has one dumb off-on switch for basement lights and one dumb 3 way switch for the stair light. At the top of the stairs is the other 3way switch for the stair light. Can I fit the Caseta dimmer in the existing double gang box to control the basement lights and leave the dumb three way switch in place? If yes, what cover plate do I need? Or do I need to separate the caseta dimmer and the dumb three way into to separate boxes. Thanks.