All the servos I ever had fail in flight had stripped gears, and in that condition represented no load whatsoever to the other servo.

OTOH if the servo motor dies then all you have is the gear train load, and the other servo can easily handle that.

I only build scale models, so most of my elevator and rudder control systems are hidden, meaning paired servos using one push-rod or pull-pull setup, and on split elevators I always use an aluminum arrow shaft to connect the elevator halves, with the horn clamped onto the shaft. Never had a failure with that layout, whereas I have had elevator failures with only one servo.