Did it, do it. The Fatboy is roomy for two up, floorboards are really comfortable, wide tires roll over anything, wind less of a problem, stability at speed is better than a Sportster. If you plan to take baggage, a Heritage is also a good choice. With a Fatboy you may need to relocate the rear turn signals for some bags. Sportster is lighter and handles better in twisties, but a Fatboy is a cruiser and built for comfort...but not as heavy as a bagger. Recommend a Memphis Shades batwing fairing to go with it.
Go for the Fat boy, I just spent 5 day's , 1,500 miles touring the north coast of England, two up with luggage, you dont need paniers, get soft luggage that goes on a rack, add a detachable back rest and rack, its cheeper than panniers.
Yep, do it. I ride two up often and it's no problem. I have mine set up with everything quick detach. I ride it stripped down with a brawler solo seat to work and around town. When it's time to take the wife for a trip. I can change the seat, put on windshield, saddlebags, backrest, and t-bag in 5 minutes.