We're using the tandem with attached trail-a-bike and kid trailer - four people and a spare seat for when the 4y/o gets tired and wants to sit with the 2y/o. You could do the same. Pix of us look pretty much the same as the above, but you can see us at:
http://arlentandem2005.crazyguyonabike.com - Actually, come to think of it, for the ride for heart we had an additional passenger in the back - the neighbour's 7 y/o -- so that's more weight than you would be looking at.