I used to use two bath developers with film a quite a bit. Everything PE says about them is true, but the variability was less than you might expect. They actually worked quite well. I have only tried them a few times in the past few years, and I can affirm that they do not work as they used to. The last paragraph of PE's original post is probably in effect. Some of the older emulsion types, like EFKE (Adox) may still work, but my attempts with newer Kodak emulsions and some newer Ilford have generally been failures.
For paper developers, I'm not sure exactly what we are talking about here. The true two bath method does work, and there are some published formulas. I've tried this method a couple of times, and it seems to work well. But why bother? You usually have good enough controls during printing so that there is no advantage, except, perhaps, with color. I did try the old Dignan divided color print system before I had a good temperature control system, and it was excellent. It had some real advantages.
The other way people talk about "two bath" developers in printing is for mixing up separate developing agent and alkalai solutions so that they keep better. (Then you combine them for a one-bath developer.) I've been using this for a few years now, and I can recommend it highly. The solutions easily last up to a year or so. Generally you mix up the developing agents with a sulfite-bisulfite buffer to keep things neutral to very slightly acid. When you add the carbonate for the working solution, you use enough extra to restore the pH. This scheme of things should work for film developers, too. But be careful here. Film development is a good deal fussier, and aging effects may be more noticeable.
For paper developers, I'm not sure exactly what we are talking about here. The true two bath method does work, and there are some published formulas. I've tried this method a couple of times, and it seems to work well. But why bother? You usually have good enough controls during printing so that there is no advantage, except, perhaps, with color. I did try the old Dignan divided color print system before I had a good temperature control system, and it was excellent. It had some real advantages.
The other way people talk about "two bath" developers in printing is for mixing up separate developing agent and alkalai solutions so that they keep better. (Then you combine them for a one-bath developer.) I've been using this for a few years now, and I can recommend it highly. The solutions easily last up to a year or so. Generally you mix up the developing agents with a sulfite-bisulfite buffer to keep things neutral to very slightly acid. When you add the carbonate for the working solution, you use enough extra to restore the pH. This scheme of things should work for film developers, too. But be careful here. Film development is a good deal fussier, and aging effects may be more noticeable.