According to me, Two Voices refer to each of mom and dad; mom has a different set of rules and principles from dad on children.

One Voice refers to both mom and dad following one set of principles and rules on children.

On Two Voices, children have choices but learn to avoid some responsibilities and likely to develop preference for one parent over the other.

On One Voice, children may learn to be responsible and treat parents equally.

In general, some children may learn to avoid responsibilities by choosing a parent figure with similar rules and principles as their preferred parent.

So a Standard Voice may opt for qualities that are most likely to be universal as a stable bind in all diverse groups. That is irrespective of difference, children are treated equally.

So when you have teachers who come out with different ideas for teaching children, it is likely to their own best interests and responsibilities befall upon them .

That is why a stable system is responsible when all voices follow one principle, the rule. But this standard is realised unworkable in different levels of disabilities and abilities.

However, specialising treatments for some children may create more stigma, but diversifying the system allows children to specialise in their various needs. They have a chance to excel in their chosen interest.

Diversifying the structure allows unequal parts to fit in.