The split control design is, as has been said, a design feature in bigger aircraft designs.
In the roll control, usually when the split is activated, one pilot has the ailerons and the other the spoilers.
Some aircraft (De Hav Canada design comes to mind) also have a "surface jam" feature where, say the bellcrank at the aileron has an upper and lower segment connected by a spring loaded locking ball engaging the crank with the rest of the ailreon control system. Now if an aileron jammed from icing or similar, the ball would ride up out of its normal location and allow the two halves of the crank to rotate independently. That is one half remains frozen with the jammed control and the other continues to rotate and the aileron control system remains free to operate - albeit minus one controlling surface.
With regard to the 717 and its DC9 ancestors, remember they are not boosted flight controls (except rudder) and the pilots are controlling the aircraft by flying the tab and not the control surface on those beasts, so the control surfaces (particulary elevators) at rest can do some strange things!