Your question made me look at the torsion axle more closely and I have now "revised" my previous answer.
If I were like some politicians I have heard recently, I would redifine the original parameters or deny I said what I said, or simply change the subject completely. In any case, I would staunchly maintain I was not wrong.
However, I'm not, I won't and I was wrong.
A tandem torsion axle system with two trailing arms must have two axle brakes. If one leading and one trailing arm axle, the leading arm axle should have brakes, the trailing arm axle should not. If two leading, either or both may have brakes.
Here's the reasoning: When you brake a trailing arm torsion axle, the brake torque will tend to rotate the torsion arm toward the rear and up, regardless of whether the trailing arm axle is the front axle, the rear axle, or both. If a single braking axle, this will transfer additional weight to the other, non braking axle, in effect lifting the braking axle. Bad.
With leading arm axles, brake torque rotates the arm to the rear and down. With a single braking axle, additional weight transfer to the braking axle. Good.
If both axles are braking, and both are leading or both trailing, there is no weight transfer during braking and all will brake equally. Good.
Finally, with one leading and one trailing, both braking, both axles will transfer additional weight to the leading arm. The trailing arm will then lift and lock up prematurely. Skip the brakes on the trailing arm.
The torsion "axle" mount is essentially fixed, and welded directly to the frame rails. This is necessary because, even at rest there is there is a tremendous amount of torsional force trying to twist the axle loose from its mounting on the frame rails.
No matter what you did to articulate a tandem torsion axle, the parts would have to be extra beefy to handle the torque. The geometry, particularly under braking becomes fairly complex.
Airstream had been running a tandem torsion axle on their trailers since the early sixties, and they are legendary for their ride and ease of towing. The difference is, the Airstream system is fully independent. Obviously Airstream is doing something right. Perhaps Moe will comment; he owns one.
Most torsion axles are not fully independent, but they do have the advantage of low profile, light weight, less unsprung weight and therefore less inertia resulting in the tires spending more time in contact with the road, always an advantage.
Still with me? I should also temper my previous comment somewhat. If you brake both axles, and pay attention to keeping your trailer level, tandem torsion axles should be OK. The disadvantage would be that most of your existing parts are not reusable; If you plan to do so you need to be careful that anything other than the wheels and tires would even fit. I would also be careful about mixing rotors, calipers, etcetera of different manufacturers, even if the differences were confined to different axles.
