I have not had two boxers before, but I currently have a boxer and golden retriever who are both spayed female. They get along great. That being said, I would not bring another female bully breed into our household. Too much of a risk of things going south with female bully breeds. All the boxer rescues and breeders that I know of will not adopt a female boxer into a family that already has another female. They might do great, but there's a good chance they won't and then one of the dogs is going to have to go which is not a situation anyone wants to find themselves in. And they will likely get along perfectly fine until the younger female reaches about a year old and that's when things will start to go wrong. At that point you are obviously already attached to both dogs, so who would want to be in that position? Obviously any two dogs can fight, regardless of the breed or sex, but it is generally considered "safer" to have a male and female boxer pair.
My female boxer is not dominant, she will allow other dogs to take her toys, eat her food, mount her, etc, etc. She has always gotten along with other female dogs outside of our home, and even female dogs who come to our house for play dates or to stay for the weekend. But then I had a friend's female pit bull stay at our place for 3 weeks. These two always got along well, have known each other for years. Everything was great for the first two weeks, then in the last week there were some changes. They started keeping a close eye on each other during feeding times (cue feeding in separate rooms), the pit started to "guard" toys and the boxer started to be interested in these toys even though she didn't care for toys previously (cue no more toys allowed), they started to compete for attention and get jealous of the other dog getting some love from me, and they just generally started watching each other very intently at all times almost as though they were each on edge or "ready" in case something started. It never escalated, but I have no doubt that if the pit stayed any longer there would have been a scuffle. The two still see each other regularly and get along great and are buddies, but they would not have made good long term housemates. I have had more dogs stay at our house than I can count (I used to own a pet care company), but this was the only time things went weird with a female, and it was also the only female bully breed that has stayed with us.