Interesting concept you have there capetonian.
Some airlines already charge 'larger' people for 2 seats.
Maybe its possible that, via the pay by weight schedule these people so charged would get a cheaper fare.
Assuming 77kg std pax and say $100 for each seat the standard, theoretically you should be permitted to tip the scales @ 154 kg for those 2 fares.
If you weight say 125kg, then you would have significant cost savings.
Maybe it is a win for our larger co-inhabitants.