I looked uo Kordon Breather bags on the web & one sight said not to dubble the bags. They don't breath right if you do.
The bag says that you can double bag. I have double bagged with great success in the past. I only do it for larger more spinous fish. The most important thing when double bagging is to not allow any water at all between the two bags. If this happens, you have effectively sealed the bag off and there will be no gas exchange.