The two phones would not look at all similar, would not have the same serial numbers or imeis or anything like that. They would be two completely different phones. The "joining" would be in the network, so that both phones ring (if both on) for calls, and calls from either phone generate the same caller-id. And of course, special billing.

This could even be implemented with the second phone having its own number that's not used for any outside purpose, other than making it easier to implement things.

Again, let me point out that you can already do this today on your own, with two exceptions. First you need two accounts (ie. it's twice as expensive) and second a call from the 2nd phone would not send the caller-id of the first one, unless you used a fancy call forwarding app. There is absolutely zero issue related to phone cloning.