That's true, but on the other hand it doesn't work with SMS messages. Well, of course you can send SMS messages from both phones, but only one of them receives them. (At least it has been like these for some years, I'm not totally sure if that's true with all the operators).

I keep running into this when some customers complain they're not receiving SMS:s from our service. It's often the case that the messages just went to the other phone.