When one column over rides they other, full control is given to the operable control column. the control surfaces are not split.
Question (not assertion), I recall training which lead me to understand that in the case of a jammed control surface, and control columns being disconnected from each other, the two halves of the flight control systems were now independent of each other (split), and the two pilots were to then determine who's controls were more effective in maintaining control. I don't recall a mechanical system which gave full control to one control column over another. This training to me was in respect of a DC-8-63. Is my recollection in error?
The Jetstream 4100 has a cockpit release to split the two elevator circuits from each other, so the left control wheel controls the left elevator, and right controls right. In the Dash 8, there is a release in the ailerons, so if one jams, it is released, and split from the other, and the cockpit controls now operate only the free aileron. This is unrelated to control columns being disconnected from each other at the cockpit.
Doesn't the "breakout box" depicted for the 737NG, simply allow the two control columns to act independently of each other to control their respective elevator?