If you have this knot completely wired, then congratulations. If you don't, then this blogpost is just for you...!
The biggest problem with the double-fisherman's is that it is very difficult to untie once it has been loaded. As a result, it is not recommended for quick situations where you want to tie two ropes together, such as in rappels.
The Canadian Guide, Mike Barter has put together the following video on how to tie a double-fisherman's knot:
--Jason D. Martin