iO HVAC Controls iO-TWIN-TR Universal Twinning Kit With 20Va Transformer IncludediO HVAC Controls iO TWIN TR Universal Twinning Kit with 20VA Transformer Included The Twinning Kit is designed to allow a single thermostat to control two identical HVAC units. It can also be expanded to control 4 identical HVAC units with one thermostat. The iO TWIN can be used on common or separate ducted systems with single stage, multi stage or heat pump equipment. Uses standard 18 gauge thermostat wire. Thermostats and HVAC units can be located