I am planning to build my own coop and would want about 6 layers and maybe 20-30 cornish cross meaties. I want to keep them separate in the coop so I thinking about duplex-like design with a human door (dutch style) separating the interior.
Winter is my biggest concern because it gets windy as my house and all outbuildings sit on a bluff, so that means drifts and increased wind chill effect, which can get to -30 at the extreme.
I like the idea of dividing the coop.
I suggest you only raise the Cornish Cross in the summer months. Since you'll be butchering them at about 8 weeks old, there's no need to keep them through the winter at all.
And then the layers can use the entire space in the winter, when they cannot go outside in a run.
The usual advice for space:
--4 square feet per adult hen in the coop (sleeping, egg laying, safe from predators, used all year long and in all weather.)
--10 square feet per adult hen in the run (daytime only, usually not as predator-proof as the coop.)
--2 or more square feet per Cornish Cross in their area
So you would provide at least 24 square feet of space for the 6 layers, for their coop.
And build a run at least 60 square feet for the layers to use in the summer.
Plus at least 60 square feet for the meaties to use in the summer, and that's also the right size for the layers to use as an alternate "run" in the winter.
For the layers: provide nestboxes for the eggs, roosts to sleep on, food and water, and some source of light (windows or electric light.) Some of those things take up floor space, so plan for that when deciding on the size.
For the meat birds: provide food and water, a source of heat while they are young, and enough light for them to see in the daytime (windows or electric light). No need for nestboxes or perches, or even a run, because they do not mature enough to use those things effectively. But do make sure the space is easy to clean. They will eat and drink a lot, and they will poop a lot, because this is how they grow so big so fast.
For both groups, make sure there is enough ventilation. Of course you don't want the wind whipping through, but air does still need to move in and out. In addition to the more usual styles of ventilation (like windows), maybe you can choose one side of the meat birds' pen that faces away from the prevailing wind, and cover it with hardware cloth instead of solid material. That would provide nice ventilation for the Cornish Cross in summer, and also for the layers when they use the space as a "run" in winter.