I actually never got a double stroller. I found strollers in general to be too bulky and not very handy and just a waste of money. I did use one for my first 2 kids but very rarely, mostly only the first 2-3 months of their life when they need to be bundled and kept warm. I found that when the babies are small the slings or wraps were more useful, then we progressed to the backpacks that dad's don't mind wearing, those can last the kids well thru the first year, usually a little longer. After that the small umbrella type strollers were easier to deal with, especially when you are alone w/2 kids.
I loved when my oldest was finally able to walk instead of being pushed/carried...your oldest is old enough now, you might want to think about having him walk more....they feel a bigger sense of self and get some freedom....they get exercise...it wears them out a bit, which is always helpful for them at naps (if your oldest still naps, all of mine were DONE with napping at 2, yuk) good all the way around...and good for your sanity in general:)
Also, most places where you will be doing a lot of walking offers strollers for renting. That's what we did anyway. Just something for you to think about.~Unless of course you are a runner...the disregard everything I said!
Another thing that might seem trivial is to buckle both carseats by a door, that way each parent can easily get one in and out of the car...it is no fun having to try to reach in and buckle the middle seat:)
I am sure you will get some great advice on here...all these momma's are wonderful!
Congrats on #2!