While I respect that others may have a different opinion, I should add that in my experience the change from 0 to 1 was more profound than that from 1 to 2. Without kids life most likely entirely revolves around your needs, interests and priorities (if you are not a caregiver). The first kid turns that world upside down. The second kid to us mostly meant more of kid stuff. Yes, I agree that it is not a good way to think about having two kids as something that equals to twice having one kid. But a lot that comes with the second will be somewhat familiar, and practice will make you better at it, even though your second kid will probably be very different to your first.

My friend wrote in an email that "When we had our first, I thought a lot about what the hell we had done with all the free time we used to have on our hands. Since we had our second, I haven't had time to think." I should add that after having their third, he has barely found time to write emails to me a few times a year.

TLDR; step size between 0, 1, 2, etc. kids is subjective.