while in the Airbus the SS movement (in degrees) is algebraically summed and the output to the flight controls is that sum
Even this definition is not strictly accurate (pedantic .. lol)
If it was really the sum of the degree of displacement of the joysticks .. when both joysticks are in the same extreme position the output (deflection) should be double that obtained with a single joystick ... which is not the case .. so it might be simply that this is a algebraic sum
if both joysticks move in the opposite direction ..