from what ive read, firefish gobies are not compatible with their own kind, let alone mixing two different kinds. if you can find a mated pair (which since you cant tell the males from females, is hard) of the same kind, more power to you, but Scott Michaels book on marine fishes does not recommend keeping firefish gobies together, but singly.
FWIW, i have never tried to keep them together so dont have any experiences to relate to you. You could try it, but one may pick on the other one and intimidate it, preventing it from coming out and eating, and may eventually 'disappear.'