A twin stroller is usually for … twins. The idea is that both seats have the identical features, thus accommodating two children that are very near or the same in age. One of the obvious disadvantages to a twin stroller is how wide it is. Usually they’re not wider than doorways, but you will quickly find situations where the width is annoying. Another potential problem you can run into is that there are six sets of wheels instead of just four. These are a little more hard to push and steer. On the plus side, with the seats being side by side you can quickly access either child without anything in the way.
In a tandem stroller, one child sits behind the other. These are easier to maneuver because there are four wheels, and they are narrower than a twin stroller. Whichever child sits in the back will obviously have their view obstructed by the front seat, but most tandem strollers compensate for this by being able to raise the back seat. Also, the back seat is usually made for the larger child, who obviously rides higher.