Why not, as an interim measure, enable the stick position display in flight (I understand it displays only on the ground)? I wonder if an obvious nose up demand from PF would then have been visible to the Captain when he re-entered the cockpit?
X-channel on monitoring pilots display only, red/amber on both displays when dual input. shouldn't be to difficult to adopt for current fleet.
But it is up to Airbus pilots to demand for such a system.
Other solutions Interlinking/tactile FB/ Decoupling/ Summation/Disabling adds complexity and far more failure possibilities.