exercise.com has a calculator which I'll assume is reliable, so I began with a BBC report taken from the ONS which says the average English male in 2010 was 38 years old, 5ft 9in tall and weighed 13.16 stone.
Okay, so the calculator says he should consume from 2090 to 2700 calories per day to maintain that weight depending on how sedentary he is. Let's take to 2400 calories for someone who is mostly standing.
Next I doubled the average guy's weight from 13.16 stone to 26.32 stone. That sounds pretty hefty to me. Going back to the calculator our not so average Englishman should consume from 3100 to 4000 calories per day to maintain his new weight, again depending on how sedentary he is. Let's take to 3550 calories for someone who is mostly standing - although that may have become less probable.
So to maintain double the average weight, our average Englishman needs an extra 1150 calories. That's not much more than a Medium Italian pizza from Pizza Hut.
From the same source, the average woman in England weighed 11 stone and was 5ft 3in tall. Let's assume she's also 38 years old. So to maintain double her average weight, our average Englishwoman needs an extra 960 calories.
To my considerable surprise, maintaining a such a huge weight as 26 stone possibly isn't that difficult. It only seems to be a pizza a day. Doesn't seem enough to me but I'm not an expert on these matters. I've never had a weight problem but I do enjoy pizza.