So this was my cheap diy solution to cool both of my systems. the pump in my veg system was a heater at the time and it kept it within a few degrees of what the flower system was set at.
The res on my flower system is a huge 55 gallon tote. It worked great for this purpose when i needed it to.
I ran another pump with black tubing with foam pipe insulation from 1 system over into the res of my flower system thats on a 1/4HP chiller. I fed it into the side of the res and split it out into a bunch of 1/2" clear tubing(cheapest and inside a closed res so not concerned about light). It stayed a closed loop and dumped back into the veg res chilled. I kept adding tubing in the loop until I was able to get my desired temps.
Eventually I coiled all of it up into a 5 gallon bucket inside my res (dont have pics sry) and let my chiller return waterfall into that bucket before dumping into the res.
So, big res on 1 system with 2 loops like this in it would work with enough tubing, the right water volume, and right sized chiller.
these are the only pics i have of it really.