I have never introduced dogs to cats so I have no personal experience, but I do know 1) the younger the better for introductions and 2) I also have seen the problems with littermates and rivalry. even to the point that the dogs needed to be separated within the house.
And even if a person thinks it would be easier to potty train 2 at the same time, it is better to focus on 1 at a time and do a good job with that and basic obedience before starting to train a 2nd.