The theory of keeping them seperate is that the meat birds are pigs and will eat all of the food, plus you don't want them eating layer feed. Also they poop a lot, and will mess up your coop for the layers (poo on feet = poo on eggs in nestboxes)
This is why I'm working on a dual purpose flock - less meat on the birds than a cornish cross, but also lay eggs and if I hatch my own, I can cook two for dinner (instead of one monster bird) without much extra expense or trouble.
If you don't have birds yet, and are space restricted, you might want to consider a dual purpose or heritage breed.
Otherwise, I would just raise the meat birds in a chicken tractor - they will be butchered in two to three months anyway, so not a long term commitment.
meri