If you want to use 2 dogs on the Perimeter Technologies Containment systems, you must purchase an additional collar. Includes Medium Comfort Contacts. For Use With: PTPCC-100, PTPCC-100D, PTPCC-100DP, PTPCC-200, PTPCC-200D, and PTPCC-200DP. Replacement collar for Perimeter Technologies pet fence systems,Soft rubber Comfort Contacts included