First, congratulations
I brought Hallie into our home when she was 2.5 and Simon was 10months at the time. She had been abused and neglected but quickly became the dominate one. I have dealt with issues of sharing, jealousy, training and yes, it's double the effort mixed with more patience than I ever though I had. We still have some issues we are working through, like her being very mad when Simon is getting love and toy sharing and it's been over 2 years!!!!! I recently hired a trainer that has been helping me by coming into my home.
I really enjoy having a boy and a girl. They have very different personalities but I wouldn't change it for anything!
I also want to say that you are doing a great thing rescuing/adopting! Way to go~