Firstly, I would like to point out that, whichever developer you choose to use that gives you the results that you want is fine by me. I just want to point out that the opinions offered in this thread regarding two-bath developers are, in my opinion, incorrect as they do not reflect my practical experience of using two-bath developers.
Two bath developers are extremely tricky to use, especially if you use more than one type of film, because no two films will pick up part A in the same way.
I am afraid that I couldn't disagree more. I have been using two-bath developers (Thornton's since he first published his formula) for more years than I care to remember with consistent success. Different manufacturer's films can be successfully processed together provide that they are if the same ISO speed rating (irrespective of personal EI used). The argument that "no two films will pick up part A in the same way" may be theoretically correct but this is not borne out in my practical experience - having processed literally 1,000s of rolls of film (mine, colleagues, students and friends).
In the case of Thornton's two-bath, I can state that it is extremely consistent, easy to make, cheap, easy to use and delivers negatives that never have blown out highlights. It is, however, no magic bullet and should be employed for normal to high contrast scenes. It is not suitable for achieving higher EIs nor 'push' processing and certainly not for very flatly lit scenes. One interesting side point is that, since I have been using Thornton's two-bath with Delta 400 (my standard film for many years), I have stopped needing to use filters for good rendition of skies.
In the case of D-23+alkali bath, or the many other variants of Metol-sulfite (A)/Alkali (B) development (Stoeckler, Leitz, Vestal, Farber, Thornton, etc. etc.), realize that all the normal variables present in one-bath development apply: Time, temperature, agitation.
You are quite correct I can never believe how many people on the internet bandy around the 'wisdom' that, with two-bath developers, they are virtually impervious to time, temperature and agitation. This is not the case. They are perhaps more forgiving than some other developers but good practice needs to be employed and this means that they need to be used (as with all processing) in a consistent and repeatable manner.
And of course with relatively short times in bath A, and relatively fast development in bath B, achieving development uniformity can be tricky.
Here I beg to differ. All of the various two-bath developers that I have used have had the same times for Bath A and Bath B. Certainly, with slow (up to ISO 50) films in Thorton's two-bath the given processing times of 3.5 minutes in each bath are somewhat short but I have never had any uniformity issues in any format (35mm 5" x 7"). However, this may well be because I always use a pre-soak and I always start developing with 4 inversions in the first 30 seconds followed by one inversion per following 30 seconds.
As I said at the start, use what gives you the results you want BUT if you are looking to try out a different developer because of contrast, blown-out highlights, grain, consistency issues then I would recommend adding Thornton's two-bath to your list.
Bests,
David
www.dsallen.de