With kids that old, your very best choice would be a Kelty Deuce Coupe umbrella jogger. Not cheap (they are nearly $400), but they do have great re-sale value on eBay. (There is a used one up right now @ $182 with 25 bids on it.)
Generally a jogger is going to work best with kids at high weight, unless you can find a Mac double, or an old Peg MicroTwin (I wouldn't get your hopes up on a used Microtwin, they are rarer than hens' teeth.)
Other possibilities are the Chicco Twin Cita @ around $120 (rated 80 lbs) or the Inglesina Swift Twin at slightly <$200 (rated for 90 lbs.) Both of them have good resale value. The drawback with these is that the steering will probably be at less than optimal performance if you are near the weight limit.
Garage sales are not going to yield too many quality lightweight doubles; the resale value on them is very good, so people use tend to use eBay to try to recoup the value. (The exceptions to that tule being the Kolcraft double or the Schwinn, but I wouldn't really recommend either one for a major WDW trip; one has short handles and the other has a bar in a spot behind the childrens backs where it can be uncomfortable.) You *might* get lucky at a local kids' resale shop, but only if you visit ahead of time and let them know what you are looking for, because the minute something like that hits the floor, it will sell.
There is one other solution if you think that your older child will only occasionally want to ride; you could use a buggy-board (or similar product) with a single. My own experience is that kids still fall asleep for long periods in strollers at age 5, so that's iffy, IMO.
Your kids are almost surely too big for it, but for other people reading this, I'll also toss this out: there is a new Eddie Bauer Umbrella-fold *Tandem* twin on the market now. This is it:
http://www.target.com/gp/detail.htm...8?_encoding=UTF8&frombrowse=1&asin=B0007W05OW
) My 3 year old is very tall/big for his age, (He looks about 5) and this stroller works great for us. We did recently look for a new double because he has wide shoulders and we wanted to give him a little more room but none of them - including the macs- were that wide. If I were to buy another double I would suggest the combi. Not that I don't love my stroller but the combi folds up much smaller. It's rated for 45lbs per seat. It pushes beautifully also. It is about $199.
I already have a graco duoglide double stroller (too heavy to take), an avalon side by side double stroller (difficult to fold and not very compact), a jeep double umbrella stroller (like it, but only got it b/c I could not find a tandem umbrella ). I won't even tell you how many single strolers I have (3) 
DH does not like the jeep umbrella b/c he thinks it's too wide. 
We ended up renting a double the last time...maybe I'll point out the cost of the rental for 9 days would pay for the stroller...hmmm..or I could have my own stroller sale! That might work! 
