I really like this leash called the Pet-Walker Plus. I bought it at a dog show recently and it's fantastic, better than a coupler. I've used a coupler with my two and one was ALWAYS dragging the other and they would just spread out so far and get all tangled up but with the Pet-Walker they don't get tangled and it doesn't let one dog pull the other dog because the two leashes aren't actually connected like on a coupler.. they both just attach to the same part. Anyways, I love it.
Here's a link:
http://www.wtpets.com/petwalkerplus.htm I believe it's sold elsewhere but that's the first link I found.