Depends on where the failed servo stops. If it stops a center the other servo will provide about 1/2 the normal control movement. If it stops at the end of its travel the other servo can only counteract the problem by maintaining the control surface in it's neutral position.
And, that's what counts!
Servos rarely fail anyway but when I had an aileron servo fail on a dual servo setup it was enough to save the model.
It took full opposite aileron on the good servo to maintain control & get the model safely down.
Radfordc's linkage method results in reduced movement evenly to both elevators. One drawback of dual servo to elevator
halves is unwanted roll when one fails, not too big a problem with many models but a real problem on some. - John.