I think there is more advantages than disadvantages, such has if one goes lame you can run the other, one dog might be more suited to the ground than the other, and if you keep bitches the bitch might break down and you will not be able to run it so you could run the other dog, also if it is good lamping weather for a long period of time it would be a good idea to give one a break and run the other. That's just a few advantages i can think of...