Two kids is more than twice as hard as one kid. Part of this is that I really want to give them both the sort of attention I was giving Margaret when she was a tiny baby, and that is just not possible. I am getting better at balancing and anticipating and multi-tasking, but sometimes they will both have a legitimate reason to want my full attention at the same time, and I find that stressful. What can I say, making my kids even momentarily unhappy stresses me out. Also, if I didn't have Jomkwan taking Margaret during the day on weekdays I'd be more sleep deprived than I ever was at MIT, because it is rare that both kids nap at the same time during the day. So yeah, if you currently have only one child and are looking to expand your family to approximately replacement rate, you should know that it feels like more than twice the work. On the other hand, there are some major compensating factors that I have perhaps been failing to mention.
Watching my kids interact with each other makes up for all the extra work. Hearing Margaret say Duncan's name in an enthusiastic tone of voice, watching her rock him in the swing, kiss him, put his blanket on him... she's clearly pretty pleased with the new addition to the family, even though it means that my lap isn't hers alone anymore. For his part Duncan has been smiling and cooing at Margaret, and he clearly recognizes her as one of the more interesting and entertaining parts of his world. I didn't think they would be enjoying each other so soon, and watching them together is more than twice as fun as just watching Margaret by herself. I'm sure that in the future there will be fights, all siblings fight, but I have every reason to expect that they will mostly get along well and enjoy playing together.