I guess it depends on how you have them set up. If you are using only one temp controller to power and regulate both of them at the same time, then they will both run the same amount of time so it doesn't matter. If you have them set up separately using their own built in temp controllers, then the second chiller will not be working nearly as much as the water coming into it has already been chilled once. This means the first chiller is working much harder and will wear out faster. Other than that, there is no reason why you couldn't just loop out of one into the other.