On the Airbus, it is steer-by-wire, so the tillers are just spring loaded potentiometers/position sensors that input to a computer. I believe Boeings have mechanical linkages to the steering? If so it will be far more expensive to fit an extra tiller on the RHS.
I love it that some folk hate the idea of F/Os taxying ! The same F/Os can land in terrible weather at night, but are not trusted to taxy very slowly along a yellow line with the Captain and ATC watching closely.
