The PetSafe Exra Collar Receiver PDT17-10726 Can be purchased as a replacing collar for the PDT00-10603 and the PDT00-10933 or as an add-on, allowing you to increase the number of dogs that can be used with the system.  These collars are sold separately.

Note:  The PetSafe PDT00-10603 Yard & Park Trainer at one time came with 2 collar receivers.  PetSafe has re-packaged this product so that it is now a 1-Dog system.....you would need to purchase additional PDT17-10726 receivers to make the unit a multiple dog system.