There are two ways you could look at this.
The first is that we already have ‘companion’ devices anyway. The iMac’s companion is the MacBook, whose companion is the iPad, whose companion is the iPhone. All we’re doing is continuing the succession with a ‘cardphone’, for want of a better phrase.
The second is that we’re getting into a ludicrous Russian doll-style situation here and we can expect a phone the size of postage stamp next, then one the size of a flea and then one the size of molecule. If we see things this way, the world has clearly gone bonkers and it needs to stop spinning for a moment so we can all get off.
I'd be surprised if this idea catches on, although stranger things have happened.