The angle between the drive shaft and the rotor hub doesn't change - how is that a CV joint? Only the blades and the plates change angle with respect to the MR shaft, not the hub.
if the velocity of the blades don’t change (it remains constant) despite the angle between them & the mast changing then it is a constant velocity joint, hence the name.
To elaborate I’ll refer back to the OP’s comparison of a universal joint & use a truck as an example - although it might appear that the drive shaft on a truck rotates at a constant speed, this is only the case when both drive shafts either side of the universal joint are inline. When the rear axle moves & the drive shafts are no longer inline, the velocity of the driveshaft rotation fluctuates twice/revolution as the centre cross of the universal joint moves. This fluctuation is minute at small driveshaft misalignment angles & increases as the misalignment increases.
The terminology of “constant velocity” refers to the driven portion of a power train, the “joint” refers to the ability of the driven portion to operate out of alignment with the driving section.
The disc does not remain aligned with the mast & the velocity of the blades remain constant, makes sense to me so how is it not a CV joint?
Back to original question, I’m not aware of any CV arrangements for 2 bladed Helicopters. Maybe the teetering head doesn’t create the blade velocity fluctuations like a universal joint would so it’s not worth the engineering costs to change it? If the teetering head doesn’t create velocity fluctuations then by definition it is a constant velocity joint & most of this discussion is irrelevant. I would have thought that an elastomeric head would have been an option, are there any around?