Does having a second child make things twice as hard?

My first child is four years old and growing well, which is great, but parenting isn't always easy. There are times when I feel exhausted, and this often leads to big arguments.
We had planned to have two children even before the first one was born, so we are thinking about trying for a second one this year, but I am currently feeling quite worn out and am debating whether it is okay to have another. My life goal is to raise two children and see them get along well, but because things have been difficult lately, I have a lot on my mind.
To those who have already had a second child: does it become twice as hard once the second one arrives? It is already tough with just one; should I go through with it?