This is how I read this passage:
A clear distinction between rationality and sentiment would be very convenient, but it is not real. I agree there is none. So as not to repeat myself I will point at this overly-long argument elsewhere: https://philosophy.stackexchange.com/a/24936/9166
We want to believe that intelligence and morality are either identical (a la Kant, or Utilitarianism where the answer is a formal proof, or a computation, and thus always done most easily by those gifted with quick thinking) or separate (a la Calvinistic religious traditions and their philosophical offspring, or revivals of naturalism that hold morality is a gift of Nature separate from reason, based in the sentiments, including a specific example of a philosopher, Scheler, who considers 'love' the basis of all morals.) But in fact it is not that simple. [Those injected examples, other than Scheler are mine, not his.]
Those who want the two closer together, also want morality to be more universal, which is in fact anti-intellectual, as it does not allow the individual combination of reason and sentiment that come together in an individual to be exercised fully. It indicates that intelligence needs to be shaped a given way, and what does not fit is to be disowned or ignored. He thinks that this is badly exacerbated by globalist notions of intelligence like those of Hegel.
Intelligence is, instead, better seen in terms of its literal synonym, Judgement, which is more obviously an active factor of morality. It channels our sentiments into action, and to the degree we attempt to isolate the two, we fail to inject our 'soul' into our morality. ('Soul' is my gloss, Adorno avoids the concept with the Freudian theory of drives.) We can only contradict this with a certain kind of 'love', in the moment, that dissolves boundaries. We can never do so by recombining reason and sentiment later.
Not personalizing our ethics and using our 'soul' makes us, to a much higher degree than is reasonable or necessary, agents of the social forces that shape our reason from childhood.
But those social forces have enough power, worldwide now that Western logic extends into part of all other cultures, that those who cannot bring these two forces together without help the social institutions will not give them, will disown intelligence, and those who could do so will avoid doing it because playing the game of keeping them separate allows the intelligent to worship their own strength.
It is hard to see this as evil, because evil would involve active negative sentiments, and simply isolating sentiment so that it either escapes logic, or gets devalued, means we can't bring those negative sentiments into action. So we can see the whole enterprise of separation as somehow reducing the level of evil in the world. But only in a quite odd way.
More realistically, we can see that this is nonsense, and the distinction is ultimately destructive, because the isolated parts of the intellect we disown, and the sentiments we devalue will find some way to express themselves. (Again he takes up a Freudian/Kleinian argument to show how this is so, which is overkill in my book because it is just obvious.)
(My general reaction to Adorno is that he constantly says something obvious. It is usually already said by Nietzsche in a way that is entertaining. But he puts it all in a way that is instead just exhausting. It is funny that both he and Nietzsche trashed the popular culture, particularly the music, of their day.)
