Hi Steph. I got my second Yorkie when my Newman was seven years old. I don't really have any advice except to go slow when introducing them. Never leaving them alone until you are 100 percent sure and even then watching for awhile. Newman took pretty good to Bella. Of course he growled a bit. I think they try to establish dominance in which by the way Bella turned out to be the boss and Newman the wimp. LOL. Newman is now Ten and Bella three and we actually added two more and all four get along GREAT. I think the first thing you need to see is if Baxter is dominant or passive. If he's more dominant then you'd want to get a more submissive pal for him. You really don't want two dominant dogs in the same house. That's just my opinnion, of course some more experienced may have more to say on this
Best of Luck to you. Elaine