Dual input indicates that both side sticks are being moved at the same time. The summed input of those two side sticks is sent to the flight control surfaces not to exceed maximum control movement.

Yes. Important to emphasize that you cannot get more than the single stick max input. Both sticks at 50% (same direction) will sum t 100%. Both sticks at 100% will sum only to 100%.

If the sticks are moved in opposite directions that will result in subtraction.

Yes, could be any combination of same or opposing inputs.

AA300 NTSB report
 

Two 58 year olds scraped a wing and almost cartwheeled an A320 on takeoff because of poor crosswind control, and were hanging onto the tail of the plane trying to figure out what happened. But by all means, let's raise the retirement age.

And then blames everything on airbus planes sucking.

Good summary, hadn’t heard of this one. Thanks for posting.

What does age have to do with it this incident?

Do you think he was a crappy pilot for decades and got lucky or do you think he was a decent pilot at one point until age got the best of him?

My bet is crappy all along. Lots of those in this business who seem to slip through the cracks.