When you get to the breeders house, if you haven't picked one out prior, and I wouldn't, then I'd have both of them brought in together and check 'em out.
Seperating them to see their individual personalitites isn't fair (imo) at such a young age for this breed, unless you're really experienced and know what to look for from each pup when they're seperate and what they're behaviour means.
Just because one of the girls may seem timid or more so than her sister DOES NOT mean that that's how she is going to mature naturally.
One of the BEST Yorkies I've ever owned is Nemo. (actually he belongs to my folks) When I saw him and his sister, I didn't know what sex I wanted at the time. I chose him for reasons unknown to me right now, BUT I do want to mention that both puppies seemed a little unsure, but curious. When I got him home, it was only a few days before the fourth of july hit. We were outside and the fireworks began. He did all he could to hide under my skirt (he was in my lap as I was sitting in the grass on a blanket) and was trembling. I thought to myself....what did I get my mom into?
Well, time went on and just through proper rearing of a puppy and allowing him to no longer "hide" from ANYTHING and putting him through obedience and distractions, his confidence level SKY ROCKETED! He now does an entire Schutzhund 1 routine-yes, including the blinds/and bark and hold; he is beyond the UDX (AKC) level and tracks like no other....he's just the coolest little dog ever. You NEVER in a million years would've put this dog at maturity as behaving that way as a puppy.
Sometimes they are what they seem. Quite honestly, but there are other times in which they're not.
SO, w/ that said, I would just go there, take a look at both pups and see if either are inclined to play w/ a toy of theirs or w/ each other. Don't necc. go for the one that pays the most attention to you. You're a novelty and as soon as that wears off, they'll go back to being a puppy. Just choose one that suits your entertainment value at the time, since it's going to be a healthy pet and not bred or shown, and get her home and ENJOY HER!
Play w/ her as much as her energy will allow and develop a fun bond. Teach her rules of any games you play and just enjoy her.
I'm so jealous. Puppies are great! Housebreaking and all!!!!