its called a "leash coupler", it looks like a "V" and it attaches to their one leash and then the splited leash has those attacher things, and then you attach it to their harness/collar. I see them at petco and petsmart. they are online too
I hope this is of some help
www.petsmart.com
www.petco.com
www.petsmart.com
www.petco.com