Hi bill1983:the two systems are independent,one thermostat control the two systems. Indeed it seems the former serviceman farly misunderstood the original problem so added more refrigerant and opened the TEV.The question is it possible that this wrong proceeder cause that the 10tons system work at higher evaporating temp that made the difference wide enough between the two evaporators that made the 10tons system to act negatively;i.e the 10tons evaporater were relatively "warmer" enough than the 15tons evaporator that caused the the thermometer read slightly higher degree when the 10tons enter work?