I work on chillers quite a bit and from my experience if it is not a closed loop they will plug up with deposits. Flushing a core will remove deposits but if the core is copper or aluminum most flushing chemicals will destroy the coil. Most of the chillers I work on have either stainless, zinc alloy, titanium alloy, or cupronicle. To do what you want you really should use a closed loop but why not just use two res's and pipe in two closed loop heat exchangers for each res with one chiller. You may need a small inline circ pump to move water for grow res loop to keep it at temp you want. There are many ways to pipe what you need with out using two chillers. The stuff I work on is quite large so may not be cost effective for what you are trying to do, if interested PM me. I'm sure i could come up with a cheap cooling piping schematic.
