First of all, congrats on the twins, the second time is easier emotionally than the first time (I am a parent of 4, 3 alive, one died in my arms after a genetic decease)
Twins are not about the emotional energy as much as it will be a matter of organization, of mutual support as a couple. And the emotional needs of your first born will increase, so, with the tone of voice of “Scar” in the Lion King: Be Prepared!
You mention “Having a child is a lot like running a marathon. An incredible challenge, but a worthwhile and transformative experience.”. Yes it is, we moved out of a self-center adulthood to find someone that we love more than ourselves. This is a good thing, it makes us better people; if couples love would be this way for both men and women, there will half or less of the divorces that there are today. Unfortunately, in today’s world, consider your kids lucky to have parents like you that are emotionally healthy enough to make the transition.
One thing I learned is that it is important to create good memories, happy memories. In the long run, that those help when they become teens, and you become failable. That is when we become Nanny MacPhee (when you need me but you don’t want me…).
And give it 3 or 4 years, you and your son will discover that are minority in the house in a very real sense. Girls might be daddy’s girl, but they will learn to read you as a person much faster than the boy, You know how your wife can sometimes complete your sentences? My daughter has been doing that since she was 5.