While an acidified ammonium thiosulfate solution will act as a bleach the action is fairly slow. Thus Haist recommends a pH of 4. The only problem is that acidified thiosulfate solutions are unstable and the lower the pH the faster they sulfurize. So the pH of 4 is probably a compromise between bleaching rate and stability. Still I don't see such a bleach as competing with Farmer's. Ilford in their book on photochemistry gives a large number of bleach formulas for the three difference bleach types, sub-proportional, proportional, and super-proportional.