This is one of those questions that I'm sure you could ask 100 people and get 100 different answers. It depends on how you parent, it depends on your kids' personalities, it depends on lots of other things.
But...I can give you my experience as a parent of two. After we had #1, our lifestyle had already adapted to working as a family unit and not as a couple. That's a huge adjustment. Also, we had one so we knew pretty much what to expect with #2. That's a generality, of course, since siblings can be vastly different. We have two girls, and they are similar in many ways. Having #2 enriched our family and just felt right. I wouldn't say that having #2 was no big deal, but I also wouldn't say that it was terribly hard. It was another adjustment. On the practical side, it is more work. There are two kids who can get sick in the middle of the night, two kids needing your attention, twice the laundry, twice the food, twice the tantrums, and IME, when you add sibling issues into the mix, it can feel like there are twenty kids instead of just two. OTOH, there are twice the kisses, twice the happy memories, twice the fun, twice the milestones, and so much more than that because you see them interact with each other, too.
I think it also depends on your own emotional health and how you handle stress. Some women just seem more capable of handling more kids. I'm not one of them.

In my house, "if mama ain't happy, ain't nobody happy." The stress must be managable. I don't want to feel that I'm shortchanging anybody--including, or really especially, myself. I feel that I have a great balance right now. I work in an office 3 x week and am home with my girls 4 x week. We have family close by for support and practical help with watching them. With two kids, there is a parent for each. And now, if we had #3, I think the stress would go up exponentially. I love our family just as it is. And I also have to add that I have a phenomenal husband. He's a wonderful father and we truly parent as a team.
Our girls are exactly 3 years apart (minus 5 days!). Generally, they get along great and are good playmates. Other times, I swear they want to do each other, or me, in. I think that kids can learn a great deal about life from their siblings, and DH and I both had siblings and felt that it was important. We initially felt we wanted three, but always planned to defer to real-life experience, and so far that's working for us.

Good luck with your decisions!