I don't think it has to be an acid fixer in order to use part B after fixing. I have been using part A, no water rinse, then part B. Then a water stop is used because I don't have an alkaline stop. After that, fix the film, pour part B back into the tank and it will stain the film more. Then wash as normal. I have had great results using this. Part A gets dark but doesn't 'die' on me, like you said, up to a year old for an open container. If you use it for more than one roll of 120, which I do, you can tell when it starts to die because it will turn green. When that happens, I make a new A batch. I use part B maybe 2 or 2.5 times longer than I use part A. I love the stuff really. Everyone thinks I have sepia-toned my negatives which kinda tickles me.