ORIGINAL: schroedm
As for movement, to my mind it requires both servos pushing equally in order to drive the pushrod fore and aft. If one servo fails I think the other servo will just be rotating the bellcrank around the 'pivot' of the bellcrank and not 'pushing' the pushrod fore and aft??
with the one failed servo being fixed, you've then changed the pivot point to that servo, that moves the push rod 1/2 the travel of the other servo at twice the torque, when you get the oppertunity, just unplug the one servo and you'll see it first hand.
the only way you'd get no deflection on the push rod is if you set the servos to travel opposite each other (one go to the front as one goes back), then you'd make this a bell crank, in the configuration it's setup as and intended it's a leaver, not a bell crank.