I had two boys that lived together peacefully. There was never any snarkiness between the two. Franny has been with three other females at the same time at the dog sitters, all were the same age, and never had problem. It basically comes down to the individual dog.
The only reason you may not want to get one the same age is that you may lose them close together. My boys were a year apart in age and I lost both of them within 9 months of each other.