I would recommend a male over a female. Two females can work, but it depends on their personalities. Same-sex aggression is a big concern, and isn't always evident at first. Some females will wake up one day from a nap and decide they hate each other. And if you aren't home to actively supervise I just wouldn't think it a good idea.
Mom To~ Dakoda (b/t GSD), Acheron (black GSD)