Take the outside of the white disc and the inside of the black circle: O. It is, as Wittgenstein suggests, nonsensical to talk about the fitting of the disc in the circle when the boundary of the disc is defined, given, by the circle. Good, yes, right. They are not related as two separately definable or defined phenomena. But then does it really help us to say that they are instead related internally?
What is it that are related here? Objects or concepts? (This is a doubly terrible question.) Well, one thing is for sure: it is not that we have here two objects which are related by being 'brought under a unifying description'. There is no more fundamental designation of these alleged two objects than 'this disc defined by that circle'.
No, the designation doesn't unify; we do better to say that we have here one figure, and that we can focus on certain features of this figure. There are two sides to a coin - we cannot peel the sides off from the singular coin to which they belong without their ceasing to be sides.
The problem with the 'internally' answer to the 'how are they related?' question is that it fails to completely take apart the assumption that here we do indeed properly meet with a 'they'. It's a bit like (to borrow again from Wittgenstein) answering 'Mr Nobody' to the question 'Who is in the room?' where there isn't anyone in the room. We could speak like that, but we both know it simply disguises the out-of-place-ness of the question.
The only reason it looks like we meet here with a 'they' is that we have started talking as if that is what we have here. But we don't, so we shouldn't. Sit yourself down in the right place to start with and you won't ever have to go anywhere.
We get this kind of thing all the time in philosophy. 'What makes it the case that 2+2 is necessarily 4?' Bad question - this whole 'makes it the case?' way of speaking already presupposes that we here have to do with two separate phenomena (2+2, and 4) which are by some or other means, you tell me, joined up. But, no, it is not the Platonic objects or the rituals of human life that 'make' this the case. Rather what is the case is that we have such a singular ritual as adding 2 + 2 and calling this '4'. One phenomenon, one rule, one coin, one O, and no thing to relate to any thing else.
How are the morning star and the evening star related? They are not related or unrelated. There is no 'they'. There is one star with two names. ... How are the two names related? ... Well, what kind of relation did you have in mind in your question which asks about how names are related? I can't yet think what you're talking about.