My story isn't exactly the same ... I knew I wanted a second, but I didn't want him so close to my first. My first child is extremely intense, emotionally, and was quite a challenge as a baby. I was feeling quite overwhelmed caring for one, and couldn't even conceive of managing two, and my husband was nostalgic for his carefree days before we had a child. I was in shock when I found out, and then I cried. I had to talk (crying) to both my best friend and my parents before I could even work up the courage to tell my husband, because I knew he would take the news even harder than I did.
While pregnant, I definitely went through a process of grieving for our old, 3-person family. I kept thinking about how moments and routines that I cherished would forever change. At the same time, early in the pregnancy I had a miscarriage scare (which turned out to be nothing) but it made me realize that despite my trepidation, I DID already feel close to and want the second child. The fears and mixed emotions continued throughout the pregnancy, but with joy gradually replacing most of the worry. My husband went through a similar process.
Its not always like this, but for us, adding a second child to our family ended up being much, much easier than I ever expected or hoped for. My second child is generally calm and happy, and was since the moment he was born. My first son, who is so attention-demanding, completely loves his brother, and showed surprisingly little sibling rivalry, and instead seemed to view his brother as another person who is there to pay attention to him! :-)
There are ways in which having two children is DEFINITELY more than twice the work than one. And there have been days in which I've felt overwhelmed or when I have hated the noise level in my house. But there are also many many ways in which having two is actually easier than having one. And, although I would never recommend having another child for the siblings' sake, I really feel that having a brother is the best thing that ever happened to my first son. My boys have such a great relationship, and my youngest son brings out the more loving and joyful side of my oldest.
Anyhow, the experience of two children has been such a blessing, that I am now pregnant with my third (and final!) child. Your fears and mixed emotions are completely normal, but hopefully you will find that the changes in your family will not be as difficult to navigate as you fear, and that your second child will really enrich your lives. Good luck!