We had twins for our second round, so we hopped straight from one to three. Infant twins are about the most exhausting thing I have ever experienced. But now that they're bigger (4, 2, and 2) they're pretty easy to deal with.
I can see how having two might be a lot of work since you could feasibly try to maintain the same level of hands-on as with one. With three, that level of commitment is just impossible, so you learn to strategically ignore your kids and let them figure things out for themselves.
It's pretty great though. Couldn't imagine a world without them at this point.