I've got 2 Boxers .. Sasha and Simba ... They are brother and Sister ... They play together, sleep together, eat together ... do everything together!!!
I wouldn't have it anyother way .. though it is time consuming to train both boxers, but its worth it!!!!!
If you do get another boxer from the same litter, pick a female ... I have heard that 2 male boxers from the same litter may have aggression issues later.....
Good Luck... and keep us posted!!!!