Obviously this Ed Lerrburg has never seen my dogs cudde together, romp in the yard together, love each other, cry if one has to go to the vet and they stay home....all that and each one still loves people attention and are the biggest love bugs you have ever seen! Yorkies are people dogs -- I don't think it would matter how many you had, they are still going to crave human companionship and loving. Some of what he says may be true -- rank issues do develop. My Ben is the boss of all the dogs and he tries to be of me at times.
But it is not a problem. The others seem to accept that Ben gets his way. I do not believe it is harder to train them with more than one -- maybe even easier as they seem to learn from each other too. The only thing that is harder is knowing who pottied in the house. But you still keep training the same if one has an accident, so it doesn't really matter who did it.
I think they do need companionship! Maybe not all the time and a single dog can definately deal with being alone during a work day for example. But I think they are happier with another dog for company. Mine do cry when left alone. They do not cry when left with each other. But then mine have had each other so they are used to that. Others may get used to being alone too. I think either way can work, but I KNOW my dogs enjoy each other and are still wonderful pets for me.
The 3 I have now are different ages, but when I got Ben, I got a female Sadie about a week or two later. So, I did train them at the same time.