I think when you have the first one done with basic training, and done or almost done with potty training then it would be a good time to get another. I would not want to try to house break 2 yorkies at the same time. If you aren't a big fan of the puppy stage you could adopt an older puppy or a retired breeder, or a rescue.
I got Maddie when she was about 5months old and she was really easy to potty train, the breeder had done alot of the work for me
She also wasn't teething her baby teeth were already in, she also was aware of doors and feet which was nice.
Some breeders hold dogs longer for evaluation, or if they are on the smaller side. This would give you the opportunity to get a dog that is beyond the "puppy" stage but still very young and playfull.