I would use two 25 foot coils in parallel. That will minimize the resistance to flow and allow the maximum water throughput. If you have the same total water flow through a single 50 foot coil vs. two 25 foot coils, you get the same cooling (other things being equal). If you could maintain the same input pressure from your tap with the double 25 foot coil as you do with the single 50, then theoretically I'd expect 4 times as much water through the double coil (2 coils each at half the resistance). But of course the input pressure to the chiller will drop as the flow increases, so just how much increased water flow you will really get depends on a lot of things (length of run from the water main to your house, pipe diameters in the house, how much scale buildup in those pipes, etc. Impossible to predict, but my gut feel is to go with the double coil.