Not only should you try before you buy, but you really need to try it with your two children in it. Since you don't have 2 children to put in it, it's not really a good time to buy. You can borrow someone else's baby to get a fair idea, but double strollers are affected by both the total weight of the two children in them and the discrepancy in weight between the two children. Some of them are impacted significantly less than other models, but that aspect will always be there. Since you don't know how far apart your children will be in age or size, it's hard to know for sure what to look for.
I'd just get a stroller for one that you love right now. If you like one that can have an extra seat put on, that's fine, but don't sacrifice too much for that stroller. If you like another single stroller a lot more, or if you can pay a lot less for a single stroller that's just a single stroller, go with what you love. You can figure the double stroller out later when you get there.
Personally, as a mom of 5 children ages 9 and under who has used several double strollers, I strongly prefer side-by-sides. I'll never buy another tandem. I'll admit that I haven't ever even pushed the singles you can add a seat to (those weren't even around when I first needed a double stroller), but I know I don't want one. I like having the older child be able to hop out on his own, and I know the older one likes to see better than that. Also, my kids are really tall, and those strollers don't look like a good option for a taller older child. Regarding Disney World specifically, many people use a stroller at WDW longer than they do everywhere else. Most 4 and 5 year olds aren't going to want to sit in one of those strollers.
Side-by-sides do look huge, but they're no wider than standard wheelchairs, so they fit almost everywhere. Only older, pre-ADA buildings have doorways that aren't wide enough. It's a a little tough to go clothes shopping in them since the racks are close together and toddlers love to touch things, but they're usually pretty easy to handle. They also fold really flat. The old-style tandem strollers tend to be HUGE when you fold them. I don't know about the newer ones, but the size and weight of a folded stroller is extremely important. There are some stroller disadvantages and quirks that you can overlook, but if it's a bear to handle when it's folded, you'll never like it.