Hi! I'm new here, going to WDW in 10 more days. I have a 4 year old and an almost 3 year old, so I can relate to your situation. I have an idea, take it or leave it as you see fit.
First of all, whatever you decide on, you need to practice at home to make your child comfortable. If you want your 2 y.o. to be comfortable in a bed, start with some practice sessions. Vacation and a strange environment is not the place to try something new, IMHO.
My suggestion about sleeping arrangements is actually more like a campout. You could set up an arrangement on the floor with a pillow and blanket. They will each have their own space, you don't have to rent something that might not suit your needs, and don't have to worry about your little one falling out of bed. And if you do this, do it at home first, having the 2 y.o. sleep on the floor of the 4 y.o.'s bedroom. My kids have done this before and they love it. Sure they may chat for a little bit but they will fall asleep.
A few other thoughts. Keep things as normal as possible for them. If they sleep with a nightlight at home, bring one with you. If they need music to fall asleep, don't forget that. My dd doesn't need it but my ds is lost without his one lullaby cd that he's listened to for 4 years.
Stuffed animals - my dd sleeps with about half a dozen different ones, so we'll just have her choose one to take with us. My ds on the other hand, has one in particular that is his special friend. It's all he's ever slept with (yes he is rather anal, lol)
Before our last vacation, we made sure to buy an exact duplicate of this item, and got it well worn in before our trip. This way, if it got left behind, there is no crisis.