What I've found is that, depending on the age difference in siblings, you now have different levels of needs between the children - one needs an infant's level of constant attention and the other needs something different. Switching between the two can be hard, especially when doing things like getting a younger one to sleep and the older one wants to do something else. The major bonus we have had, especially if the siblings get along well like my two boys, the development you were used to with the first is now different as the second has a role model to follow.
There is more work, it's definitely not proportional, since trying to find things that interest both children can be tough when going out, family outings become a bit tougher though it can be done if you manage it right. Pretty much everything will then depend on the maturity and developmental differences between the two kids, eating out, holidays, travel and so on but you also have the benefit of experience with the older one in knowing what to do when. Signs to look for when falling down, walking and toys - we have been better at choosing toys for our youngest after having gone through so many with our older. Of course, if you also have two kids with the same gender, and still have the olders clothes then you have to do less shopping as you might still have many of the things you need.