I think another Yorkie would be very lucky to get a forever home with you! Definitely more work than 1. I assume financially everything would be set. I'm sure you're willing to deal with all possible behavioral issues that could come up (even if pups didn't get along and they had to be separated). And I also assume time isn't an issue.
My girl hated other dogs for years. She got a brother. Still gets really snippy with him at times, but overall loves him. Fairly certain if we had gotten a girl, it wouldn't have worked (or not as quickly anyway). But that is her and her preference. Doesn't mean another girl wouldn't work for you. That said, why not try a Yorkie meet up and see if she does well with a particular dog and pick the sex that way? I mean if she has never shown dog aggression before, either would probably be just fine. My vote is for a boy though.
FWIW, we thought that E's attitude toward other dogs surely indicated that she would hate a sibling. No so. I'm almost guilty for not getting her one before. She can be a dog now and play doggie games with him. She is less clingy to me and seems to be a more balanced pup.