How many times do you reuse the developer? Originally Thornton wrote that one litter is enough for 15 rolls, but as the first bath really is a developing phase, there must be visible changes if developing time is not adjusted or refreshed.
I use Thornton's two bath but mainly when I only want to develop lot of bulk negatives that likes compensating nature of the developer.
From time to time I have thoughts that I'll dump d-76 and use only Thornton's d-23 variant but it would require so much studying and trials that I haven't never gone fully to Thornton's two bath.
I guess that in critical work at least first developer should be used as one shot?
Bath A should not be used as one-shot because this is just wasteful and unnecessary. The main thing to watch is Bath B which should not be over used. Bath A is more than capable of processing 30 films. Bath B can only be relied upon for consistent results up to 15 rolls.
Personally, I mix 1 litre of Bath A and 2 litres of Bath B decanted into two 1 litre bottles. I use one bottle of Bath B for 15 films, then dump and then use the second bottle for a further 15 rolls. Bath A is used for all 30 rolls.
I am sure that you could process even more but, with the chemicals being so cheap, I do not take the risk.
I have used the above system for many years with consistent results.