I think it may depend on the breed concerned.. for small dogs it may be acceptable, in my breed it is not acceptable to have a brace chain,or any sort of collar and lead and are shown on individual loose slip leads, .. partly the idea of showing dogs in brace, is to show control as well as conformation, a bracing link defeats that object..
It is quite easy to show brace, you are doing exactly the same thing as you would a single dog, (says she whos dogs went off in different directions in one brace, and rode piggy back in another

:-P )