I agree that if your 6yo is tired enough to need a ride, he'll probably want a seat and not a step. I thought that those add-on steps had to be removed, too, to fold the stroller, but maybe I'm wrong. They do stick far out and are hard to not kick while pushing. I don't think it would be worth it for a Disney trip.
Two singles would be a great plan. Your 6yo will likely need a stroller more at the end of the day/week than at the beginning, so you could just take one stroller on the early days or in the mornings. If you don't want to buy a new stroller, how about taking your P&T (without the doubles seat) and the Quest?
And FYI, you don't normally have to fold your stroller on the monorail. But even if you are at a monorail resort, you'll use busses a lot for other parks and will have to fold the stroller to get on the bus.
Single mom to
DS ("twice exceptional") - September 2002
DS - February 2006
DD - July 2009
DD - July 2009