We just returned and rented a double stroller the entire week for one child. The other half was for our backpack etc. We did say that if next year we bring our niece we would rent two doubles and again use the extra space.
All of the strollers were immaculate however we found the strollers at MGM to be older so they appeared a bit more worn. We rented for the entire week up front and that made the day start a bit easier. Once you have entered a park for the day and you have your stroller stub stamped, you only need to show your stamped stub at the next park should you go to more than one that day or leave and return.
In Magic Kingdom one day, we decided to take the train to the other side of the park after renting the stroller. We were told to leave our stroller at the main station and pick up another stroller at one of the other stations that were empty. You can tell which ones are empty as the name tags are gone. This worked great.