Interesting, I hadn't even thought of this. This will be the first time(to my knowledge) that console companies will be supporting 2 online scenes at once.
As for the chat, wouldn't that be an interesting way to show off cloud computing? 360 talks to One and the signal is received at the One with it's lower audio quality. One talks to 360 and server side computing does a quick conversion before sending it to the 360 in a way it can understand. Alternatively, just use a damn codec both systems understand.
That being said, they can't do that because they need to push the message that One is sooooo much better than the 360 they can't even talk to each other.
Btw, I'm not saying One isn't a much better piece of hardware than the 360, it is, I'm just saying that the need to push units will create artificial instances of separation between the two consoles.