My understanding is that a confirmed second appointment does not mean a reserved dose for you, as Scott pointed out. Initially, many providers were doing that, but it just gets in the way of the roll out. States and local authorities are getting better at coordinated ordering of the vaccine. The confirmed second appointment simply saves your place in the appointment system. When Texas was freezing we had appointments cancelled in California because of delivery delays.
Another thing to keep in mind is that these appointments are not like regular medical appointments. They are essentially appointments to get in line. As they open the availability, the more crowded it will be. Plus, it all depends on what is going on at a given time. My time in line was about 20 minutes for my first shot, and an hour for my second one on Sunday. The line was out the door and down the side of the building by the time I was done (twice as long as when I got in line).
In the grand scheme of things, I have been waiting a year, so what is an hour here and there?
Jan 