Re your "domino chain" idea...
Having made some batches of charcoal (larger diameter material, to make lumps for cooking) and biochar (fine twiggy feed material, to make fine char pieces), my feeling is you may be trying to solve a problems that in practice doesn't exist?
I think - If I understand the proposal correctly - that you are trying to avoid wasting feed stuff to heat the retorts?
When making biochar, I usually end up with some pieces that are too coarse to work well as biochar feed. And conversely, when making charcoal, I usually have fine pieces that burn too quickly to be included in the charcoal making process. These wastes are ideal to use as that initial fuel load to get the process started.
All that said; I have more recently moved away entirely from retort based approaches to an open fire in a trench, that is continually topped up with brash and then quenched. I can process far more finished biochar in the same time, compared to using retorts, at the "cost" of reduced efficiency. The idea is that the the continual flame front above the bed of embers protects the biochar, which builds up in layers. The resulting char is typically fine, friable, and plentiful.