I weigh about 350 and my wife is about 100 pounds lighter than I. I started of on a Honda 1100 city cruiser and eventually moved up to my current mount, the RSV. Aside from an Ultra-Classic and the BMW 1200RT we've not had many issues with riding two-up. I practiced in parking lots for about the first 10,000 miles before I ever tried riding with a passenger. The BMW was just plain hard to keep up even with just me on it, and the Ultra Classic is well... not made for fat people. I agree that the passenger should be lighter than the rider, and I agree that riding with more than the OEM-rated combined weight is a risk. For that matter, riding is a risk. Now that I think of it, every day that we live we're at risk of dieing. You have to decide whether or not it's worth it for you.
From a scientific perspective here are the risks that I can think of when exceeding the OEM-rating for weight capacity:
1. Braking - the brakes are designed to stop up-to the rated weight at a safe rate of deceleration. Any more weight than that and you're going to have a harder time stopping (which I do experience when I ride with myself and even more so with the wife)
2. Center-of-Gravity - this one can simply be compensated for, but you have to get used to it and build what sports coaches call "muscle memory." That is to say, you have to train you body to AUTOMATICALLY compensate for the change in Center of Gravity, or you will become intimately acquainted with the effect of gravity.
3. Stress on the Frame - this one is not as big of a concern unless you REALLY overload the bike (like try to bring home a load of gravel on the bike) because the OEM builds the frames to handle MUCH more weight than two fat people can produce.
4. Wear-and-Tear on Suspension - Primarilly shocks and tires. This just means you'll be buying more shocks and tires. Or you can do what I do and just trade the bike in for a newer one every 30k miles or so (kidding). By the way, on my Honda 1100 I averaged 8k miles on a single Dunlop 404 series front tire, but 14k miles on the same series rear tire. I have to assume part of that is because of the fat-guy in the captain's seat.
So my point is... yeah... forgot where I was going with that.
Nevermind.