Not sure about that. As you say, you are supposed to talk to them a lot and show them things to develop their knowledge of the world and language etc.
Undoubtedly, now that you have two, you are not going to be able to give ds the same amount of attention that you did, but on the other hand, he whould be getting to the age where he can amuse himself for a few moments. Also, although you aren't able to give him as much attention as you did, you now have a dd and they can amuse each other to some extent.
I do know from experience that subsequent children are easier than the first, perhaps because they start off with less attention and are more used to amusing themselves. I have 4 children and none of them were as demanding as ds1 was, but I don't think that is because he is difficult but because he had undivided attention for the first 18 months of his life.
I do sometimes feel we make a rod for our backs when we have our first child - you see everyone making the same mistakes and you wonder about it. My ds1 was my worst sleeper, for instance, because every time he woke up, we went to him. The other children, we ignored to some extent and hey presto, they went back to sleep all by themselves. But try telling a first time parent not to go to their child when s/he wakes in the night!
Sorry, I am rambling and probably not helping at all.