Welcome to YT. May I suggest you join the YTCA? Also they have some good information on Yorkies.
www.ytca.org. And one of the first things you might note, is it is their recommendation not to breed females under 5lbs.
There are also breeder members of the YTCA listing, that might know of a small study/breeders group that could be local to you.
It sounds like the first order of business is to get a very full health check done on both these two. I am assuming you got all their previous vet records, and Cerf certifications, LP assessment, prior BAT test results. Last brucellosis test results?
Have either or both of these two been Chic'd yet - on the canine health information registry?
Do you have the health lineage history for this pair?
Just curious why do you want to breed Yorkshire Terriers?
I always point new members who wish to breed the YT to our Sick and Emergency section. It is a very good eye opener on the health problems with-in this breed. And there are more than a few.
There are articles in our Library here, one very good one is How to find a Reputable/Responsible breeder.