I will just throw this idea out there:

The annual AppleCare bill on an iPhone Duo is almost as expensive as an unlimited T-Mobile data plan for an iPad, $240/year vs. $200/year.

You are correct in saying that yes this is one device that roughly is equivalent to an iPad mini, but in terms of cost, durability, screen quality, etc, you have to really find the iPad mini to be unworkably difficult to carry around to make the cost and fragility of a foldable make sense.

There are benefits to having two devices in addition to the downsides. Two people can use them. Two batteries versus one, double the run time.