Except for twice the expenses, I think it really is easier having two. They keep each other occupied and seem happier. Like normaansburrell wrote, they seem to pretty much teach each other. The rest of her advice is good, too.
Mine always walked in sync, as well as went to the bathroom in sync. I always paid a bit more attention to the resident dog just do he doesn't get jealous. The only problem I've ever really run into is when one eats much faster than the other, then steals the slow poke's food. I use one of those fancy slow down bowls, and it's fixed the problem.