So many misperceptions.....
I've been putzing with divided development for years. I like the "Zone System for roll film" ability, so to speak. I never got the negative density with Diafine that I want, and have moved on to other formulations. But here are some facts about Diafine and its use. Except as noted below, agitation really has no place or bearing in divided development in either solution.
1. The Darkroom Cookbook has a Diafine copy; it is a P-Q formula. I have no knowledge if it is indeed a copy or just gives similar results.
2. The pH of Diafine Bath A is 8.25. This is WAY below what will give any development, even with Phenidone, unless you left it in there for hours.
3. The Bath B is about 10.5. The MSDS show that sodium carbonat is a component, I'm guessing probably the only one. Five grams per liter will give you 10.5 pH.
4. As some here have said, "no" development takes place in Bath A. All we are doing is getting the emulsion to accept developer. Agitation means nothing in this step, overlooking possible air bubbles. Agitate continuously if you want, or just let it sit. Or as I do as a CYA, every minute or so just to make sure there are no bubbles preventing absorption. The warmer the developer, the faster this will happen.
5. If Diafine is a PQ developer, or even MQ with highly active ratios (1:9 and 1:4 respectively), the Hydroquinone is more active at the higher temperatures. This means more contrast as the temperature goes up. It is not a true panthermic developer, although much more so than traditional developers.
6. Agitation in Bath B should be minimal to none. Agitating undoes the very characteristics we are looking for in a two bath developer. As someone pointed out, at the least, it will wash Bath A out of the emulsion. Also, it will bring fresh developer action to the highlights, which will cause less shoulder. While certainly the box suggestions are a starting point, try using less to none at all.
7. Time in baths is of minimal consequence. If you want some control, try using different temperatures (both baths the same!) If the developer is cool, there will be less swelling of the emulsion and hence less developer to work with in Bath B. If your negatives are thin, try developing at 75 or 80 degrees. More time in Bath B pretty much just causes more grain; more time for the silver filaments to grow without any corresponding increase in density. Someday I'm going to do some experiments on this matter but I would wager that 95% of the development is done by a minute in Bath B.
In my two bath work, I pour in the Bath B, rap the tank onto the counter a few times, then let it go to work. Never a bromide drag problem with my current formula, which has a lot of potassium bromide in it. If you can get away with this with Diafine, by all means do it.
Of late I've used a two bath by that old school photographer-journalist, Otha C. Spencer. (Does anyone know if he is still alive?) I kept an article by him from 1992 where he tells about his formulation. Since I've not been able to enlarge my negs, I can't yet say anything about grain, but he claims 11x17's "without grain." (Film speen unstated.) But I do get very printable negs from two stops under to two stops over the native ISO! Two minutes in each bath and it's done.
Mi tres centavos....