What has Airbus done to address the dual input issue?
Nothing was changed in the stick design or logic.
The side stick design and logic are not addressed by the final recommendations of both accident reports. Of the issues were the training and lack of crew resource management (CRM). Recommendations toward the manufacturer, for example, are with regard to the barrage of warnings the crew of Indonesia AirAsia had, even though Airbus aircraft already have an EMER CANC button to mute spurious alerts:

The KNKT recommends that Airbus to consider in developing a means for flight crews to effectively manage multiple and repetitive Master Caution alarms to reduce distraction. (PK-AXC accident report)

(There are already buttons to take priority, as well as aural and visual cues as to which seat currently has priority.)
Active sticks
Coupled digital side sticks weren't there when Airbus first introduced the A320, as a technology they became feasible in the early 90s. And were first introduced in c. 2015 on the Gulfstream 500 and 600 business jets.
Imagine panicked, under-trained, and uncommunicative pilots in an active sticks cockpit; they might think their sticks got jammed or the coupling motors are acting up. And seeing how long it took for the active sticks to make it into a business jet already hints at the snail pace (and cost) of aviation hardware certification and approval in general.
As for the captain's / left seat inputs being correct, as others have commented, there have been numerous accidents where the captain was in control and the first officer's recommendations were not heard or they kept silent, again, bad CRM.

Re comments on visually noticing the other yoke (not really a side stick design/logic issue):
That did not help Flash Airlines Flight 604 and Atlas Air Flight 3591. Tunnel vision and/or not speaking up is the main problem. Same with the thrust levers; despite moving automatically on Boeing planes -- showing the current setting -- they did not help Asiana Airlines Flight 214 and Turkish Airlines Flight 1951.