Why on earth would you want 'arithmetic summation' of both sidesticks ?
The Airbus control philosophy seems counterintuitive, byzantine and needlessly complex with no consideration to human factors or simply
'whats it doing ?'
And for all its vaunted precision I read many accounts of its lag and the difficulties this causes in gusty crosswinds.
I don't believe it was outside the state of the art for the sidesticks to be linked and backdriven when the A320 was introduced and I know it wasn't an issue for the autothrottles to have had the same arrangement.