ADMIN EDIT: Please note this post was accidentally submitted in this thread instead of Turning my co2 extractor into a cryochiller?! Gas expert needed! :) - please disregard 
I would suggest something to function as a restriction point between the high pressure extraction vessel and the lower pressure separators. The expansion point can be a needle valve or a small nozzle to restrict the flow and allow for lower pressures to be maintained in the separators. The lower the pressure you run on the separators the lower the cooling temperature will be. You will still need a water chiller to run a closed loop cooling cycle in your system. The heat from compressing the separators to higher pressures has to be displaced somewhere for the refrigeration cycle to continue. Your chillers need to be able to hold a steady 5c under load for you to get the best performance from the co2 cooling.
Your separator’s jackets will function as your heat exchangers. Pump the extraction solvent through the separator jackets flowing top to bottom for best exchange. Running your separators at around 250psi should cool your solvent down to -15f, 140psi should cool down to -30f