Nice device for close range/smaller dog park training. Nice customer service. Got a second dog after some time of being a single dog household and was pleasantly reminded that the unit was expandable to 2 dogs. Sweet. So we purchased the 2nd receiver and collar and violá. Tips: purchase a collar that is not the same as the black original. That way, you can match the black button to the black collar, and the red button to the second collar. Also, for the not-so-tech-inclined, watch a random YouTube video or two on how to pair the second collar and you will be good to go.