Intimacy is the indwelling of one substance in another. Because of our corporiety, we can never perfectly achieve this. We can share something of our substance though its operation, though this only occurs though the material medium of signs and so is always to some extent extrinsic. We are impeded from intimacy by differences and ambiguities of language, the limitation and inferiority of signs and media of expression, the corruptibility of our cognitive organs, etc.. The private character of our subjectivity arises from its imperfection.
Angels and blessed souls overcome this imperfection of subjectivity by immediately illuminating others with their thoughts, desires, and works of art. They cannot be divided from each other by diverse languages, the ambiguities of one language, or the limitations of a medium. They create art immediately in others without having to manipulate paint, tones, words, or other material. Nevertheless, no angel is the source of the existence of another. If, per impossibile, one angel were to cease to exist, no other one would do so (which is also true of human beings – my son does not die because I do). The angel and the blessed soul* does not depend on things lower than himself to make himself known, and the activity of his subjectivity is can be entirely intimate to another. Nevertheless, his proper existence is always isolated from the existence of another.
It is only the trinity that definitively overcomes all impediments to intimacy. The spirituality of God removes the impediment of corporiety; and the procession of persons in divinity is such that if, per impossibile, one person were to cease to exist all other persons would also immediately cease. Each person exists within the other by a mutual and complete indwelling, in the same way that the will is both entirely contained within the intellect (as its power of desire) and entirely contains it (by being able to move it to act).
*The damned angels and souls are such precisely because they have rejected the intimacy of mutual indwelling, as laid out in the two highest commandments.