Something to keep in mind if you do decide to get a double from offsite. It is not just weight you have to worry about, but height. We were at Disney in December with my 5 and almost 8 year olds, and the big one had just come out of a boot for a fractured ankle a week before we left. I knew there was no way she could walk all day on that leg. We ordered a CitiMini GT (I think) that by kids together were well under the weight limit. But she was way too tall for it, her head rested on the folded up sunshade which she said was uncomfortable and she didn't always want it pulled down cause then she couldn't see.
Also, while under the weight limit, pushing a combine 80 lbs of children around WDW was NOT my idea of a good time.