Wait, I am slightly confused here now
Is this supposed to be a moveable tractor or is it meant to stay permanently in one place.
If it's a tractor, I'd say that if you don't have something mechanized to pull it (and good ground on which to do so), it would be a real challenge (but not absolutely impossible) to build big enough for 10 hens. If you want to try it, I highly recommend making the house and run as two detached structures, to move separately and then hook back together.
If you want a tractor but don't want the challenges inherent in building a hand-moveable 10-chicken structure, then two separate 5 chicken tractors makes perfect sense. As you say, something like 3x4' houses, with as big a run part as you can do. However if you do that, you can still perfectly well use 2x2 for most of it (I'd use 2x4 for the 'sill' that lies on the ground) to make it lighter. Rip yer own 2xss, tho -- the construction-grade ones are worthlessly crappy, and the 'knotty or select' grade spiff ones are unreasonably pricey.
But if you don't need a tractor and will be content with something that stays in one place, don't build two half-size things; build a single large-enough (or too-large) structure. It will be as cheap or cheaper, and approximately half the work to maintain. Really!
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If you use a metal roof, bear in mind that you'll be wanting either plywood or insulation (peck-proofed with thin plywood or whatever, if it is in reach of the chickens, as it would be in a tractor) on the underside of your roof. Otherwise you will have lotsa condensation/humdity problems as the cold metal condenses out air moisture in wintertime.
I'm also going to have the nesting boxes off the sides of the thing, so that ads and extra 6-8 square feet to the overall coop size....
I wouldn't really say that nest boxes add to the coop size from the chickens' perspective; and it is often possible (especially in a full-height rather than 3'-high-tractor-style coop) to work them in somewhere in the existing coop. Mine are under my droppings boards for instance
Also I am planning on having two rows of roosting poles per box. stacked styles.
I am not certain what you mean here, but recognize that if you put one roost actually *above* the other, half the flock is going to be seriously pooed on, and seriously p.o.'ed about it, and there will likely be HUGE squabbling for the top roost (even more than normal). Really if you have the opportunity it is probably best to just make all roosts the same height - reduces the incentive for arguments (which can lead to bloodshed). Remember it's really important to all chickens to be roosting on the highest place available, they don't like being second banana
Good luck, have fun,
Pat