In the past I’ve wondered whether it’s possible to use a single Garmin Forerunner 305 GPS trainer watch with two Garmin GSC 10 cadence sensors (i.e. with two different bicycles). The answer is, yes, you can; however, the device can only work with one sensor at a time. When you switch from one bike to the other, you have to enter Settings -> General -> Accessories -> Cadence Sensor, and from there perform a rescan, so that the 305 will pair with the other cadence sensor. So, you can only be paired with one sensor at a time.
It should be noted that when you pair the 305 with a cadence sensor, you should be well away from the other bike. If you try to pair when you’re physically close to both bikes, you’ll get a “Multiple Cadence Sensors” error as soon as you select “Start Rescan”.