We had a similar problem - we use a 20 bbl cold liquor tank in our walk-in to cool 10 bbls of wort. When we started doing double batches the walk-in couldn't chill the summer 80 F water down fast enough for the second batch. We made a coil of 50 feet of 1" copper, immersed it in the cold liquor tank, and tapped into the glycol system for our fermenters. There were only about five joints we had to solder. Luckily we had some excess capacity to do this on the chiller. It was a cheap solution, much cheaper than a double heat exchanger and new glycol chiller.