Are there certification requirements on Dual Inputs?
Here's what EASA Certification Specifications for Large Aeroplanes CS-25 says about design for dual controls:


  CS 25.399
  Dual control system
  
  (a) Each dual control system must be designed for the pilots operating in opposition, using individual pilot forces not less than –
  
  (1) 0·75 times those obtained under CS 25.395; or
  
  (2) The minimum forces specified in CS 25.397 (c).
  
  (b) The control system must be designed for pilot forces applied in the same direction, using individual pilot forces not less than 0·75 times those obtained under CS 25.395.


And in many places the requirements state that it must be demonstrated 


  ...that exceptional skill is not required to control the aeroplane


The A320 complies. It is designed for control of one pilot at a time, like all other aeroplanes. There is no requirement for a dual control system to be mechanically linked. The B737 columns and wheels can be uncoupled after an elevator or aileron jams, the remaining control provides input on top of that of the jammed circuit.