I've thought about this a little bit.
United, Austin to NYC (Economy) costs ~$330. So, two standard seats on that flight would be ~$660.
United, Austin to NYC (First Class) costs ~$1440.
So, for the price of FOUR Economy seats, you can sit in first class. Or, for the price of half of a first class seat, you can get more space to occupy.
Legally, if they're going to require fat people to buy two seats, they're also going to have to allow thinner people to do the same. Else, it's discrimination.
Sounds to me like they might not have thought this one through.