Hi C.,
My son was 22 months when my second was born. Although it was pricey (I shopped around because the prices varied), I got and like the Combi Twin Savvy LX. It's a side by side that fits through regular doors. The seats don't recline quite all the way (even though the website says they do), but I found that with a blanket, the baby fit fine. I also used a front carrier with the baby and relied on my single stroller for my older son for a while. Some of the Combi stollers also let you put their infant carriers on them -I don't have one, so didn't try that part.
I like this stroller because even with the weight difference, it didn't pull to one side and it only weighs around 20-25 pounds with no kids. It doesn't have great baskets underneath and no cup holder for mom, but there are other places to store stuff.
I used it almost everyday during the spring, summer and fall last year around my neighborhood and it was a great way to get out of the house with the kids.
If you can borrow a stroller and a kid to try it out with, that might be a good bet. One thing I found when reading reviews of strollers is that for every one, a group loves them and a group hates them.
Good luck with the kids!