I got a coupler, but gave up! I walk mine on seperate leads. We have also started taking them on individual walks once a week. Ash has become a terrible teenager and started running off and causing havoc in the corn fields, something he had never done before. Now he stays on the lead while Forrest is allowed off when they are together over the fields. On his individual walk over the fields he is allowed off and it gives me lots of one to one training time with him. Forrest loves his individual walk, normally he is off over the fields and just checks in now and then, but when I have him on his own he never leaves my side and we do all sorts of little training exercises such as roll overs and twirls. It is time consuming, but really makes a difference.
I suppose, even though dogs get on well, they must get fed up of each others company and need a break.