A bit in a hurry right now, so a quick answer with several links.
There are three options for connecting several tableaux supplémentaire here in France, these are the single-phase (monophasé) examples:
Model I: standard approach. Only one RCD, serving also the coupure d'urgence function if this tableau supplémentaire is not inside the main building.
Model II: if the second consumer unit has more than one RCD and/or is not inside the main building (main breaker not easily accessible), you'll need a so called interrupteur d'urgence (also: interrupteur sectionneur}.
Model III: three more or less separate / autonomous consumer units, incoming supply from one main breaker, the disjoncteur de branchement. All cable sizes from the main breaker should respect the maximum current of your DB, the courant assigné. For these currents use a répartiteur, in a coffret de répartition. A simple mouse click is enough to calculate the minimum cable size.