I'm sometimes jealous of people who only have one and are happy with having just one. I believe that on some level, life would be so much simpler and it would be, like Issymum pointed out, like being me and DH with a child. Having a second child seemingly balances the scales out ; 2 parents on one side, two parents on the other side. But young children are so needy that it's not really balanced. 2 young children in the house feels like more than double the 1! But, I know it won't always be like this.
But, having a second child was always something I envisaged and I didn't make that decision primarily for DD1's sake, I made it for my sake. It's not like we even thought about a pros and cons list. 2 children were how we envisaged family life. One child wouldn't have done that for us. Just as I realise that 3 or 4 children are what some others envisage as a family.
It took me 12 months to conceive dd2 and with 2 miscarriages inbetween dd1 and dd2, we did go through a few months of wondering whether dd1 was going to be an only child. And if that had happened, we'd have embraced that and looked at all the obvious positives that brings.
However, as I have said, when we thought about having dd2, it was an unconscious decision.
We love having a second child. We don't see any negatives. But I suspect your dilemma is similar to the one I now have over whether to have a 3rd. ;) That is something over which I do the pros and cons and come out with no correct answer.
One thing I worried about when I thought dd1 might be our only child is jealousy between DH and I. Only because anecdotally I had witnessed friends who were only children have a peculiarly close relationship with their father and no happy relationship with thie rmother. It was like the mother and daughter competed for their father's attention. Since my DH is a very hands-on dad with our dd1, I was scared that I would be the one left out
However, being aware of that fear, I'd have made sure we each fostered an interest/hobby to share with dd1 separately.
Could I write any less concisely?
Good luck with whatever you decide. Don't have a second baby just because you feel you ought to. There are no cons to any family size if the children are loved and taken care of.