I would change the DTH to this z-wave tweaker and check to see which parameter is set for the external switch right now. Change it to 2 state model. [BETA] Z-Wave Tweaker
As for why 2 switches and not just one. I think because this module supports so many type of switches and also auto detect when first installed. It requires 2 inputs. Again, I am just guessing so you have to contact Aeotec to confirm.