Rosie was 4 years old when we got Mikki. At first it was a little rough but now they play and love each other. We still have our moments, you know they are both females, but all in all they love each other!
The other thing I need to throw out there is to keep in mind it s double the expense. It is double everything. But if you can afford it, then go for the second one!
One more thing, if I had it to do over I think I would go with one male and one female. I wouldn't trade my two girls for anything in the world. But I think a male and female would make the transition a little smoother!