My sons are 18 months apart, and we have the Baby Jogger City Mini. I had every intention of getting a tandem, but was ultimately turned off by the length and it also seemed like there was very little room in the back seat. The City Mini fits through any door that's handicapped-accessible and is even easy to steer with one hand and has lots of other great features. I rarely use it now that my son is almost 3. Since your kids will be 2 years apart, you might want to consider borrowing a double or getting one used, especially if you get a pricey one because your 2 year old will soon prefer to walk than ride in the stroller.
Tandem is best because side by sides do not always fit through a door when you are shopping.