For starters, I have no idea if this is still the case. There were rumblings last year that they were only doing the individual pizzas with the dining plans.
Now, the original design, and what may still be (again, I have no idea if it's still true) was that 2 people would use a total of 2 credits to purchase the larger pizza, and get the proper other bits of the plan along with it. So, 2 total, not 2 each.