David,
From your reply it seems that this behavior is what happens today, and it ill not change for NNS any time soon, it will just run on https://identity.ic0.app and nowhere else.
Here is what is not so clear from your answer:
1.- It seems an on-chain DAPP can not support both domains correct? That would explain why the NNS only works with ic0.app.
2.- It seems however that a client app, a JavaScript interface could theoretically connect to anywhere. Correct?
3.- How can a regular user have a single Internet Identity (a single number) that connects to both ic0.app and internetcomputer.org domains? It seems impossible using the normal process. On the link you shared about [Juno] you said it was possible to have an II number work in both domains, how?!(Internet identity and login - #2 by peterparker)
When registering an anchor on the Internet Identity, it can work with both domains, but it requires some configuration. That’s why your anchor may work with one method but fail with the other.
Thanks,
Joseph