adding a second baby?

I've always wanted two kids close in age. My first baby is almost 18 months and I'm thinking now is the time I know sibling bonds (if they're close) are irreplaceable however I'm having a hard time because I've nannied my whole life and I've always felt closer to one of the kids than the others. I really like giving all my time and attention to one. Is it different when they're your own?