If you set the device ID for a channel, it should connect to that device regardless of the presence of other sensors in range. How it actually works is beyond me, that magic is handled in the ANT chip. If there was a massive amount of interference from other sensors in the area, it is possible the chip could be unable to find the specified sensor, but it really shouldn't be an issue.
If you are paired to your heart rate monitor and in a race corral with hundreds of other runners also wearing monitors, your device can still pick up the sensor. That is using the same mechanism.
