It sounds like you have things going in the right direction. When you give them attetion of any sort whether it's affection, treats, food or play make sure you give it to the boxer that was there first meaning the boxer that you got first and then the other boxer second and make sure you do this every time and that everyone in the house does the same thing and this helps establish the alpha between them because they already know your are the top dog
I hope this helps. Good Luck !!!