Yeah, couldn't you just run the two evaporators in series and use a suction accumulator? What about an external equalized TXV with the evaps in series? You could also use a high concentration calcium chloride brine solution as a secondary refrigerant with a single evaporator, as the eutectic point at 30% mixture is ~ -50 deg C, but the pump requirements will be high since the viscosity is significantly higher than water. Not to mention it may corrode things. A 60% mixture of ethylene glycol will also give you freeze protection down to -55 deg C and you can get that at walmart in the car section. Depends on how cold you want to get, but I'd imagine going below that in evaporating temp would require more exotic equipment and refrigerants, and you have the temperature difference between the actual boiling liquid and the surface of the heat exchanger in contact with the glycol as a buffer.
But I agree that you're going to end up with a mess and have no idea in h3ll what's going on with it and why it doesn't work right if you have never messed with or built any refrigeration systems before. Start with something simple so you get an idea of how it works and what affects what before adding more factors and variables. It's like trying to understand advanced mathematics without mastering algebra. Edited by RADEON - 9/6/12 at 1:39pm