I personally don't think going from 0-1 was harder than 1-2. But, I don't think 1-2 was harder than 0-1 either!
I simply think, they are different type of transitions. Here's my experience.
Going from 0 to 1 was a total lifestyle change. I went from being selfish, doing whatever I wanted with my time, to having to prioritize someone else 24/7. Everything was new, and a bigger lifestyle shift. Although I personally was very ready for it, so it didn't feel overwhelming to me & it did come naturally.
But with a first child, I did allow some of society & people's opinions to sink in, I believed some myths, which indirectly didn't allow me to listen 100% to my own intuition.
But on the bright side, with one new baby, I could actually rest, work or do whatever I wanted when she slept. I could take turns with husband and the baby. And I could take the baby anywhere and do adult-like activities and have conversations. I could adapt to her schedule as she grew. It was a much easier transition in that sense.
Going from 1 to 2 was easier in the sense that I already knew exactly what I was doing, I had more realistic expectations, and I wasn't worried one bit on doing things MY way. I listened to my intuition 100%, I didn't listen to people & their opinions, which made it all less stressful and helped go with the flow. It was so smooth!
However, having a toddler was the HARD part the second time around. I couldn't be cuddled during the newborn stage, I had to be active & take care of a toddler's emotional needs. I pretty much forgot I had a newborn. I had to adapt to TWO schedules, and there was no me time left. Taking turns with my husband typically meant having one kid myself.
I truly can't say which transition is harder or easier. They each had pros and cons.
BUT what I can def say is that, HAVING TWO KIDS IS HARDER THAN ONE. Ha! Although they can play together and entertain each other, having two kids is simply.... NO. TIME. OFF.
Don't let that scare you.
Just being honest with MY experience.
Equally more fun, more love, and I absolutely LOVE it.