Yep, another one here who could have written the OP, about 2 years ago before DS was born. DD was my "little buddy" and as excited as I was about the new baby, I was apprehensive about not having as much time for her and not being able to focus on her as completely as I had before.
2 years later, I can say that some of those fears were founded and some were not. It is harder to give the first child as much attention as you did. Unless you clone yourself, nigh on impossible (especially if you have a very clingy 2nd, as I did/do). To some extent, I think that's actually good, because as an only child myself, I can testify that it's not always great for a kid to be the only star in the sky.
But it's true, at least for me, that with 2 (or more) you have to work harder to specifically nurture the close connection with each child as an individual. I have to specifically make time for me to be one on one with each child. In early December, DD (now almost 4) and I had a day out together in NYC, and she was just aglow. It's hard, but I try to work time like that in as much as I can. I also work hard to resist the temptation to "grab just a little bit of work time" or clean up the house during DS's nap (kids are in day care and I work 3 days a week, then home Monday and Friday) and instead just focus solely on DD during that time.
The other thing is that I have to realize that what each child wants is my undivided attention. Sure, they want it all all the time, too. But for my kids, especially DD now, half an hour when I'm totally present and involved with her and the game we're playing or book we're reading is way better than an hour when I'm in the room, but focusing on a dozen different things. That is an easy lesson to learn intellectually, but an extraordinarily hard one for me to put into practice, because I'm a multitasker by nature. But I'm trying!
It's amazing to be a family of four, though. Our house feels so much more full and warm and happy. Sometimes chaotic and out of control, too, but whatever. And the joy I get when I see DD and DS together--when he wakes up and immediately asks for his big sister, or when she runs to comfort him when he falls on the playground, or when they are just giggling hysterically together--is just overwhelming. I love having my arms full of both of them.
By the way, I'm having the same fears about #3, due in June, as I did with #2! Especially since DS is so all about Mommy...we'll see how I divide myself three ways! The fact is that while your love may expand infinitely for each child, your time does not. So yeah, it does all work out...but not without working at it!