For a long time I felt like I could only be a good parent to one child at a time.
I found the transition of 0-1 easy. She wasn't a unicorn child, who came out sleeping or anything, but it was a happy, relaxed period in life. Going to 2 was much harder - its much much more than double the work, you have far less downtime etc. The first few months were fine, because i just strapped my newborn to me, and kind of got on with life as before. When both were mobile, that's when it really got tricky!
A little while ago, I got an out of the blue devastating diagnosis for my youngest. Before being transferred for life saving emergency surgery, I had to make 2 calls. One to tell my husband (child's father) and the other to make arrangements for my other child.
Obviously this is a very extreme example, but basically no matter what the situation, your mind cannot be fully on one child.
A lot of people surfing struggle with their first, to find time to eat, shower etc. With a second, you have to FIND that time, because toddler life doesn't wait just because theres a baby. If they need the toilet, they aren't going to wait until baby has finished feeding etc.
On days where we've just had one child, it's like a genuine day off, in comparison to having both.
Its wonderful having two, our two love playing together and they bring so much joy. Its totally worth it, but it is far harder.