While the codes may have changed a lot since the '70's when I was a journeyman electrician, I seriously doubt that there is any rule in the national code about having more than one circuit in a box. In fact, in my day it was very common to run a 14/3 romex to a recep box on the far side of a house and split off two 15 amps circuits from there. Dishwashers and disposal circuits were almost always done that way -- 12/3 to the switch, and one leg would feed the dishwasher and the other the disposal.
Here in the Villages, I can almost guarantee that the switch leg from the switched receps in your living room is on a different circuit from the ceiling fan in the same 3-gang box. I know that my own house is wired that way.
But you bring up a very important point that a lot of amateur "electricians" may not be aware of. If those two circuits are on opposite legs of the 220 service, you could be playing around with lethal levels of power without knowing it.
I can tell you from experience, you haven't seen fireworks until you've seen a 220 volt dead-short in your face (I had a blind spot and picked copper specks out of my face for a month when a screwdriver slipped as I was working on a hot breaker box). And while you have to try pretty hard to electrocute yourself with 110 volts, 220 will typically kill you instantly.
I almost got to experience that one myself, too. I was working on an air conditioner on the roof of an apartment building, and I specifically warned the woman in the house to not touch the breakers while I was working on her A/C. I bet you can guess where this is going. I was standing in a puddle on a flat roof, pulling the wires through the conduit when this moron decided to start flipping breakers after she overloaded her garbage disposal. I had the grounded conduit in one hand and all three conductors in the other hand at the moment she threw the switch. Fortunately, most of the power blew up in my hand from the dead short of the three conductors. It tripped the breaker (and I was a 20-year-old kid), or I would never have survived. It still felt like someone had hit me with a sledgehammer in the chest. It knocked me right off my feet, and blistered both hands and the bottom of my feet inside my rubber-sole boots.
Every electrician I've ever known could tell you stories like this. If you don't know what you're doing I would advise you to leave the shocking stories to the professionals!
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