If your two breeds of chicken crossbreed, the offspring will not be purebred. And the offspring of those offspring won't be purebred, either--you'll have yourself some beautiful mutts. However, the original parents will still remain purebred.
Hello and Welcome To BYC! X5 As long as you separate out the flocks into the correct breeds about a month before you want purebred chicks, (and keep them separated as long as you want to collect purebred eggs) you can run them together at other times... A lot of people run their breeding birds as one big flock in the winter and then separate them again in the spring when they want to have purebred chicks. Being bred by a different breed doesn't change the genetics of the female, either in chickens or in dogs, they can both produce purebred offspring at a later time.
Welcome to BYC! Glad you decided to join our flock. The first chickens I ever had were Light Brahmas (about 50 years ago) and they were gentle giants. My children made lap pets out of the later Brahmas that we had. Please feel free to ask any questions you may have. We are here to help in any way we can. Good luck with your breeding.