I can see why you may want to do this if you need to save a channel.
But given the 30.00+/ea price per servo you are citing you would be talking about high torque minis with metal gears.
So you will end up with about the same amount of torque with a good standard size high torque servo.
That equals out pretty well.
Weight wise, things will be about the same.
The downside is that now you have the CF rod hanging in the middle between the two rudders.
As it is I have to pass my electric starter UNDER the tail after starting the plane, and the CF rod would make this even harder to do.
From a drag standpoint, it would be debateable as to which is more aerodynamically efficient, but I'd expect them to be about the same.
So I see things of equal value, and one negative point.
What am I missing that would offset the negative point?