I never kept them together, but I've kept both before and I wouldn't advise it. A dragon goby will not live to its full potential in fresh water and a bichir will not live to its full potential in brackish. They simply have different requirements. Even if you ignored that and kept them together in full fresh I think the goby would be dramatically out-competed for food and most likely bullied and harrassed by the bichir.
My $0.02
My $0.02