The average (fat) American man is 5'9 and 195 lbs. The average (fat) American woman is 5'4 and 166 lbs. Therefore, we could expect the average (fat) American couple to be somewhere around ~370 lbs. So your estimate that together they would be a combined (fat) 350 lbs isn't that far off. If this couple was (fat) average, they would still have another ~100 lbs before they reached the 450 lb weight limit for the ride. The fact is that they are even more fat than the fat average.