I don't have tons of yorkie experience, but I got a female and she was 10 weeks younger than my male. They did the same thing for the first few weeks. Actually they continue steal each others toys

I think it is their way of claiming ownership.
Whenever my male would growl at the female or try to nip at her I would tell him no and lay him on his back and tell him no again. When she did the same thing to him I would do it too. I think I read somewhere on here that it is important to establish that you are the pack leader and neither of them hold that role.
It only took a a few weeks (3-4 actually) and they are now the best of buds.