Hmmm.... interesting to read about the problems fitting older kids. On our last trip to WDW, we travelled with friends who had a folding double stroller (Graco?). The toddler sat in front, and the infant rode in the back. It worked, but the parents commented that it was very hard to steer when the toddler was sitting in it. It was also a bit challenging boarding Disney buses, with a tired toddler in one hand and folded stroller in the other. (The mom carried the baby and the bags.)
The only double strollers that fit a variety of kids sizes well are jogging / biking strolllers, but they do not fold well and would be a nightmare on Disney busses.
Based on this and the earlier posts, you may want to consider getting a reliable single stroller, preferably an umbrella stroller, and only renting a double stroller on the (hopefully) rare occasions when your 5-year-old is too tired to walk.
Select an umbrella stroller if you can fit all your gear into a small backpack (diapers, camera accessories, snacks and water). Select a full-sized stroller if you need more storage or can't cope without stroller cup-holders. Keep in mind that umbrella strollers may have lower handles, and your back will feel the difference.
Here's what we experienced taking different strollers to different parks with our DD (one kid only, so far):
My wife and I took our full sized folding stroller to WDW, and treated it like an SUV. (It has four cup-holders!) We carried our typical diaper bag kit, plus extra jackets, voluminous shopping bags, a handful of resort mugs, and a baby! We also cheated and used a rental car most days. (We rode the bus to MK because it's faster than driving and taking the monorail.) It was also faster for us to push our stroller between the car and the park gates than it would have been to empty the stroller, fold it, and use the parking lot trams.
However, we downsized when we visited Tokyo
Disneyland and Tokyo Disney Sea (and other sites around Japan). We used a Tigger umbrella stroller we found on sale for $15 at Zellers (similar to K-mart). We used one or two backpacks to carry our diaper kit, snacks and extra clothes. This was very convenient for getting on and off buses, and when we were in the parks, we hung our bags on the stroller handles. (Of course everything would tip over backwards if we took baby out without remembering to unload the handles.)
Which is better? It's like deciding between and SUV and a mini cooper. I'll pick the mini. My wife leans towards the SUV. DD doesn't care.