Running more than one yard
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You split locations when one place stops feeding the bees you have, not when you run out of room to put boxes.
Why a second location
Forage is the usual reason. Colonies in one yard draw from overlapping ground, and past some density they compete with each other and with whatever else is flying nearby. The yard does not announce this. It shows up as lower average production per hive while your hive count goes up, which is why you need per-hive numbers rather than a total.
Risk is the other reason, and it is underrated. A yard is one pesticide event, one vandalism incident, one flood, one bear. Operations that keep everything in one place eventually learn this in a single night.
How far apart
Far enough that the forage does not overlap, which in practice means a couple of miles rather than a couple of hundred yards. Honey bees will fly several miles when they must, so separation is about typical foraging range and not maximum.
Against that, every mile is time. A yard forty minutes away gets visited less than you intend, and the colonies there will show it. The honest planning question is not how far you could drive but how often you will, in July, after work.
Getting permission
Most out-yards sit on somebody else's land. Get it in writing even when the landowner is a friend, and put the boring things in it: who has access, what happens if the land sells, who is liable if someone is stung, whether you can drive in when it is wet. Honey as rent is normal and is not a substitute for the written part.
Check the rules before you place hives rather than after. Requirements vary by state and sometimes by county, and several states require registration of colonies by location, which means a new yard is a filing and not just a drive.
What a yard costs you in time
A yard is not a fraction of your attention, it is a fixed block of it. Loading, driving, unloading, suiting up and packing down happen once per visit regardless of whether you open four colonies or fourteen. That fixed cost is the argument for fewer, larger yards, and it is also the argument against putting six hives somewhere scenic and far away.
The practical rule most operations land on: a yard should hold enough colonies to justify the drive, and you should be able to service it in one trip without coming back for a tool. The second half of that sentence is what a checklist and a standard kit are for.