How to Keep Both Boxers Happy
There is typically a transition time in which the dogs will have to get used to having the other permanently there. While they may have gotten along nicely when meeting, now that they are in the same house, they will be feeling each other out and needing to get a few things straight.
Who's the Leader -
The most important element (touched on above) is that while the human must be seen as the one true leader, among the two Boxer dogs, there will also be a leader. One Boxer will be more dominate than the other.
It may make sense at first to assume that the established one will be in charge. However this is not always the case. Also, some assume that the male - no matter his age - will take dominance over the female; this is also not always true.
In most cases, the Boxer that was there first - and especially if he is older - will be the one.
Most dogs will figure this out on their own. There may be a transitional time when one tries to take a toy and the Boxer that really wants to be in charge will put the other in his place with nudging or a snap in the air. Only intervene if it appears it will endanger one of them. With most, there will be a bruised ego and nothing more.