You can also try this. Check the profile of the person who uploaded on youtube. Seem he claim to invented it and has brought him some fame. Clearly it's 2 Schwinn S1000 welded together, so i don't see the big deal on his so call ingenuity but from the video you can see it still retain the seat post on both scooter, so there's no stopping you from keeping the seat on and make enough space for 3. Not sure if it'll be able to carry your weight. Each schwinn s1000 is rated to carry 260 pounds so if the total weight of the family is less than 520lbs i don't see why not.
http://youtube.com/watch?v=3fBAJr2kVLk
