There was a member on here who had a dog with back problems from using a coupler. It puts a lot of stress on their little bodies. I bought one for my two but have never used it.
We walk with two separate leashes. It took me a LONG time to get used to it but it works out for us now! Maggie tends to get tangled easily so we use a retractable with her and just a regular nylon leash with Franklin. They sometimes tangle and switch sides and you can see me twirling and turning with my leashes while walking down the street.
