I've been using this Forum as a great resource since getting my yorkies in July. Because I know lots of people go to this for information, I just wanted to post an update on the situation above that I mentioned.
The aggressive behavior got worse and worse. And it didn't just revolve around food anymore. Moo would violently attack Puck whenever she could. Their last fight ended up with Puck's left eye almost being taken out (he had puncture wounds and had to be rushed to the vet to get stitches). So Moo went back to the breeder, and we eventually had to find another home for her.
I have spent so much time wondering why this aggressive behavior happened. My mom has two yorkies, and they've never had fights like my two did. In fact, I have quite a few friends who have sibling pairs that don't have aggressive behavior. I think I have finally figured out what the difference was in my situation: there was no other (older/established) dog in the family prior to their arrival. My mom has a jack russell who was there for at least 2 years before the two yorkie puppies arrived. Same with my friends who also had 2 puppies. There was always an older dog who essentially was the head of the house.
This article goes into some good detail about the situation that my husband and I found ourselves in.
Giving up Moo broke our hearts. We love them both so much, but I could not run the risk of leaving them alone and having another (potentially much worse) fight break out. Puck is doing well, and we're in close contact with Moo's new parents.