I have thought of this fantasy myself. My wife has always been about 100 lbs ahead of me, and we have put on about 70 lbs each since we got married.
(We actually know a couple - not well, but enough for the odd visit - and they have got to be around what I would love us to be. She is mid-to-high 400s I would guess, maybe peak 500+ but she has lost a bit, and he is definitely in the 400s but I am not good at guessing men's weight. What I would not give to see them going at it!)
On the one hand, it would be hot to get to around 500 lbs, and I love to eat, so it would be fun to do. On the other hand, getting that big is bound to get a lot of comments and "well meaning advice" on being so big. I would hate that, but I could live with it. Biggest thing that holds me back are what it would do to me physically. Not just BP and diabetes, but my joints ache enough (especially in the morning) at 260 that I cannot imagine what doubling my size would do to them. Even putting on 10-15 lbs (my general fluctuation range) makes a difference.
But that does not stop me from fantasizing about being a huge couple!!