A refrigerator appliance is provided including a freezer chamber divided into a first compartment and a second compartment by a removable mullion. A first evaporator cover is positioned within the first compartment to define a first evaporator chamber and a second evaporator cover is positioned within the second compartment to define a second evaporator chamber. A crossover duct is defined in the mullion for providing fluid communication between the first evaporator chamber and the second evaporator chamber.