It really depends on the dogs and their personalities. Here is a pic of Emmie Lou who is now 7 1/2 lbs (having a hard time getting rid of that baby weight or the vet says it's really just less food needed..lol) and Fufu who is her daughter and almost 3 and half lbs and 10 months old. They get along perfect, but both are very easy going
I have another almost 3 1/2 lb yorkie male and no problems there either