Can we say that the wellbeing of one should trump the other? How can we chose which it should go?
On this thread we have talked about a lot of very different situations, in and out of school, and a lot of different kinds of behaviour, so no one answer is going fit every case. But I don’t think it helps at all to talk about any child’s needs “trumping” another. We have to maintain a balance between the needs of all our children, as parents or teachers or friends.
And schools do that (OK not always very well). On the one side, My DS's school told me very clearly that if his behaviour could not be brought under enough control then they couldn’t keep him because it would be too dangerous and too disruptive to all the others. (With help he did stabilise, despite some wobbles.) And on the other side of the balance, my DS’s funded TAs didn’t just support him directly, they also helped other children deal with him, and they kept an eye that they were all safe together and not upsetting each other. When DS didn’t need his TAs right there they could also spend extra time with other children who needed other kinds of help, which compensated a bit for the extra demands he put on the class and the teacher.
we are asking children to do that by including other children who have different behaviours, some of which they won't like at all
Ideally we would not just ask other children to do it and then leave them to get on with it. Sometimes children need adult support to include those other children as friends and schoolmates, so that they can be safe and so it's good for them and not a burden.
There are things that your DCs school could do better and your DC2 might really need access to a social skills group so he can make some friends there and also learn to interact with other kids at school. And where I live your DS1 could be seen as having additional needs because of his brother's condition, which might be taken into account at school, or with other support like a social club for sibs of kids with ASCs. I realise those things may not exist where you are and even if they did it might not be enough, but even so it's about general under-provision and not about who “trumps” who. 