Two shear walls of very different stiffness can end up carrying an even 50/50 split of lateral load, or a heavily lopsided share — depending entirely on how stiff the floor connecting them is.
A diaphragm is the horizontal floor or roof plane that collects lateral load (wind or seismic) and delivers it to the vertical elements below — shear walls, braced frames, or moment frames. It seems reasonable to assume that a stiffer shear wall should simply attract more load, but that's only true if the diaphragm itself is stiff enough to force every wall to deflect together. If the diaphragm is comparatively flexible, it can't enforce that — it just spans between the walls like an ordinary beam, and each wall only ever sees the load tributary to it, no matter how stiff it is relative to its neighbors.