Well a sprint ring by itself has the effects of 10% Cast Reduction and 5% AC Delay Reduction.
Only one ring would work with the set, but a second ring would still perform it's normal function. Get it?
Edit: Essentially what I'm saying is this.
With the whole sprint set you would get 13% Cast Time reduction and 20% AC Delay Reduction. By adding another Sprint Ring you would have 23% Cast Time reduction and 25% AC delay Reduction. If you wear two Clock Tower Manager carded headgears you can get 33% Cast Time reduction and 25% AC Delay reduction, which would essentially be equal to wearing one Berz and a Kiel Card, for reference.