Why would I have two wires feeding one smoke detector?
If your smoke detector was 240V, then you would have a hot from each leg of your service powering it from two breakers.  There may or may not be a neutral, and there should be a ground.  The two breakers would need to be on top of each other (most breaker panels alternate legs) and should be tied together so they can't be operated independently.

If the smoke detector is 120V then I doubt it's being fed from two circuits and might have made an error in tracing.  The best way to know for sure is to measure the voltage with a multi-meter.