two female pitbulls living in the same house in need of advise

Good evening all, i have had a bridle pit who was 8 months old when we got her as a rescue. her name is shilo, and she just turn 3 in may of this year. shes always been social with other dogs and outside while playing. She is a off leash dog as well. responds well when commanded to stop or come or do something. Generally speaking there has been no issues in training or anything else. As of recently about a month ago a young man who lived next door had a brindle/blue nose mix pit and he was locked up. When this happen we took Diamond in fear that she would be placed in a shelter or worse put down. We kept Shilo and Diamond separated for 7 days. Then began to allow leashed in the same room. During this time i began to walk with my 16 year old son Diamond to off leash train her. Which she took to quickly. On day 10 we took both females out for a hour walk on leash. They did the pulling and race type walking. We repeated this for three days multiple times a day until they got use to each other. then we finally dropped both leashes and off they went into the field near our home. Running and actually playing without incidents. We have only had two times where they actually had a fight that we had to break up, thank God no serious injury, just walk was cut shorter and both dogs where placed back onto their leashes, which you could imagine they were not happy about. Now this is where i need help and guidance. They do fine until we cross the front door! Shilo who is the dog we have had for 3 year becomes nippy and wants to start a fight. Diamond realizes that she is new so she becomes submissive. she turns away and doesn’t even acknowledge Shilo. she will even walk away and leave the room. however shilo is relentless and wants to start a fight… how can i get these two to abide in the same room without issue with an adult there. as i realize we will never be able to leave them alone fir their safety. i want to keep both of them as they are so loving and are my loves and babies as my 16 year olds. please help with advise in how to get these two to get along in the house just the same as they do outside if not bettet. please help the loving pitbull mommy! thank you all