Congratulations!
I have 3 kids and one on the way, and they are 2 years or less apart. My youngest will be 19 months when the next one is born.
Here's some advice - first, the closer they are to 2, the less they will seem like babies. 18 months usually is a big turning point in their lives, and they really start to seem big. Transition your oldest to a booster chair at the table, a big kid bed, and away from anything else that the new baby might use. The sooner you start him on big kid stuff the easier it will be for you and him. As far as sleeping together - kids usually sleep pretty hard, and they get used to each other's noises, so while you might have a few rough nights, they will start to sleep through disturbances. Also, I love my Graco double stroller - it's fairly light weight, folds easily, and fits in the back of the van. As far as diapers goes, it's really easier than you think. My two oldest were potty trained shortly after 2(I think they saw the new babies in diapers and tough, I am done with that!), so I didn't have much time with 2 in diapers, and even the time I did have wasn't that hard. You find a way to organize and manage your time. The hardest thing for me with going from one to two was nursing. I did a lot more than I thought I could nursing my second!
Remember even though they are a lot of maintenance(feeding, changing etc.),newborns are essentially blobs for the first 3 months, and don't need to be entertained the way older kids do. They are very portable and don't talk back yet either :)!
Enjoy and try not to worry, it really does all fall into place quickly.