There is often need to calculate two conservative thermal combinations (lowest possible assembly temperature with highest working temperature and highest assembly temperature with lowest working temperature). Now it is needed to prepare two models with different ambient temperaure and responding case. Being able to set ambient temperature to each thermal case would simplify this procedure.
You can do this in a single model by defining the two Ambient conditions as Thermal Cases and combine these as required. Changing the program to handle two wouldn't be without its problems as the material properties assume 70F(21C) and handling the automation might cause problems. As this is not a common problem the proposed workflow above seems adequate.