Under normal circumstances when they are being exercised together, the dogs will probably remain quite close to each other, especially if they are naturally pack hounds. It can sometimes be difficult to train the newcomer without threatening the established routine which you have built up with your first dog. In the early stages therefore, you may prefer to exercise the dogs separately so there is no risk of conflict. Certainly, the newcomer should never be let off the leash before the basic commands have been mastered in the hope that it will simply follow the example of the older dog. While this may occur to some extent, there is also a strong possibility that the dogs will head off and go their separate ways. The newcomer may disappear into the distance and prove almost impossible to catch up, if it has not had a sound basic training.
If you are taking the dogs out for a walk on the leash, it is still advisable to keep them both on your left-hand side, rather than holding a leash in each hand. This emphasizes a consistency of approach, and with larger dogs, prevents you from being pulled in two possible directions, blocking the pavement as a result. It may be helpful to fit the bigger breeds with harness-type leashes, which can give you more control over their movements. Do not underestimate the power of these larger dogs if they choose to pull while on the leash. You could find it a major struggle to restrain both dogs effectively.
Exercising the dogs on neutral territory is a good way for them to become acquainted with each other without friction. Even dogs which have never met each other before will frequently walk happily together without any signs of conflict when they are outside their domestic environments. Problems may arise if you decide subsequently to play a game with them, and there is only one ball, for example. Always encourage the dominant dog to chase after it first, and then allow the other dog to participate in the game. While it is possible for two dogs to chase the same ball without conflict. there is a real possibility that this could lead to fighting, especially if the normally dominant individual is slower to reach the scene of the action.