Going to 1 child is a big life change so I'm not sure why going to two would be the same, as you have already gone through that
Because not everyone finds having their first child hard, or it a big life change. There were occasional moments yes, but I found having one child easy. I honestly referred to maternity leave as a long holiday for me.
Having two was a lot more work though (2 under 2.) Again, not initially as I personally find babies really easy, but a mobile baby and a toddler, followed by a toddler and a preschooler is a whole new world.
Take something simple like a trip to the park. One child is easy. One and a tiny baby is ready. With two mobile ones, the 16m old toddles off in one direction, your 3 year old runs off in the other direction. You need to stay with the toddler, but obviously still have to supervise your 3 year old. 3 year old gets into troubke and is screaming for you. You pick up toddler and run across the park. Toddler is now also screaming as you took them away from what they were doing. You put them down to toddle whilst your rescue older one. Meanwhile, toddler has gone as far as they can in the opposite direction and is trying to climb up a slide, although there are kids trying to come down it. Your preschooler is pulling you backwards to stay because they have now got your attention and want you to play with them. Etc etc.
Or you are having lunch in a cafe. The preschooler announced they need the toilet NOW. With one, it's frustrating, but you just abandon food temporarily and go...
But you can't, as you have two. Your 1 year old is fingers & face deep in baked beans. You don't have time in the approximately 2 minutes before your preschooler wees themselves to clean up either and so you have the 1 year old under 1 arm, your preschooler under the other, and whilst you are assisting your preschooler on the loo, your 1 year old is busy painting their beany fingers everywhere. Or rubbing them on the toilet floor and trying to lick them, whilst you plead with them not to, whilst holding your preschooler in the big girl toilet seat.
Having one can seem hard because it's a lifestyle change. Having two IS hard because you're not an octopus.
Remember the trickiest stages with having a baby/toddler - say the sleep regression - no chance of a catch up nap in the day whilst baby sleeps because of your older one. Of if you are unlucky, a toddler sleep regression at the same time as a baby one. Or you might have one early riser and one late to bed child, so you are on the go from 5am -10pm...
Saying that, once the toddler years are over I think/hope 2 is easier than 1, but before then, yeah it's harder. You are doing two different and conflicting jobs simultaneously.