I think you should wait. 10 months is too young still. Why not enjoy this great time with your dog? He will still go through many phases and changes and you should have the time and energy to deal with that.
With that said, I don't know your current situation but if I was in this situation I would wait a few more years and focus on making this dog the best companion I ever had!
Also, how is housing situation? How old are you guys? I only ask because if it is very difficult renting / city living with one GSD it is twice as hard with two.
Someone once told me: going from one dog to two is tough. Going from two to anything else you just need to buy more bowls
Also, it is commendable that you guys want to rescue and I have a ton of respect for people who rescue. Just wanted to point that out.