SportDOG Sporthunter 1800 Add-A-Dog Collar Receiver (SDR) can be used with the SD-1200, SD-1800 and the SD-2000 to add a second dog or as a replacement collar receiver. The Add-A-Dog Collar Receiver (SDR) can also be used as a replacement collar for the SD-2400. Control 2 dogs with this SportDOG Sporthunter 1800 extra collar.