That's the plan. One cooler should do both jobs quite easily, hence the need to shift on the extra one.
I don't know what kind of sustainable temperature drop I can expect with tap water. I guess it will be a compromise of flow vs temp drop so a bit of messing will be needed to figure it out.
Yes, it would be pretty extravagent just for wort chilling but I can't think of any other way of speeding up chilling in the summer.
Once I do some testing I'll report in the Equipment section.
By the way, any suggestions on counter flow vs immersion?