I have two boys, and I love having two! I had Jake for a year when we added Jackson. Jake is very much a mama's boy, so he did get jealous at first. But, once he realized how fun it was to have a buddy to play with, he didn't seem jealous anymore. In fact, Jackson is the jealous one.
I think as long as you still pay attention to your first one, she will be fine. I set aside some time just for me and Jake in the beginning so that he could see it was all gonna be okay, and Jackson wasn't gonna take all the love and attention away lol. In no time at all, they were running around together and having fun. Now they don't like to be separated at all. They are always together.
Of course, in the beginning we did keep them separate when we weren't home to supervise.