I added my 2nd yorkie about 1 month after I got my first, they were both puppies at the time.. and just recently, we moved in with my husband and his parents and they have a bigger dog about 3-4 years old and they actually accepted each other, the boys didn't fight at all, just a growl here and there once in awhile, but they are all best buds now
I like having 2 yorkies because they keep each other company when i go out to work/errands etc
i guess it depends on the dog though, but like someone mentioned above if you have the time to spend with them both, i think it's okay