Why would the cold feed be backfeeding hot on a hot water heater?
It is possible that the 1 water heater is pressurizing the line and using the other water heater like an expansion tank. I have seen homes that did not have check valves or pressure regulators have this problem when the water heater heated it pushed the excess pressure back so the inlet would get warm. I believe there was also a post where this cost the home owner because the water meter doesent count back flow but with the water heater heating and cooling and no other water usage the post said it increased there water bill until a regulator or check valve was installed.