I havnt had much experience with Boxers except for two, one we dog sat and he was a great dog, crate trained but he was extremely excitable and required a lot of exersice. He played very well with my girl dog, he was a gentlemen.
The other was my aunts secretaries dog and man she was a firecracker. A ton of energy and really needed strict obediance and even more exersice.
I have had a lot of Dobies in my life and they are such honorable, regal, gentle dogs (except the one that bit me when I was little but it was my fault, he was a trained guard dog and my dad told me to wait in the car.....which obviously I didnt, good news is after the owner came out, cleaned my lil wound and introduced me to the dog he was nice) Anyhoo my pit bulls 2nd favorite play buddy is a doberman, his name is Hank, his owner sent him away to obediance school and he is AWESOME! he is the sweetest most gentle dog, tolerates a lot, is a absolute goof and dosnt play rough with my girl at all.
You have chosen two string willed dogs that alone need a great deal of training. I figure if your going to pay that much for a dog then the proper training is required anyhow. I think they would do well coexisting, especially of the opposite sex. I have found that a majority of the dobies I've encountered are not animal aggressive and IF anything they are the peace keepers. I wish I knew more about boxers in that sense but I'm afraid I do not.