Why do I need to turn off two breakers to cut power to an outlet?
It doesn't matter if hot and neutral are the same wire gauge (the breaker must be for the smallest gauge used). I have a bunch of 10AWG neutral on long 20A branch runs, because I had a surplus and wanted to reduce loss.  If mixed, they absolutely must run in the same cable or conduit - it is not acceptable to use a hot from one Romex and a neutral from another Romex.

Are you sure you're not looking at capacitive coupling?  If a dead wire runs alongside hot wires, it will pick up a "radio signal" from the other wires, and that phantom voltage can be detected by a very sensitive meter such as a modern DVM.  Usually something will seem "a little off" like it will be 10V lower than every other wire.  The proof of the pudding is to hook it up to a night-light or use a lower-impedance voltmeter, the voltage will vanish like a ghost. 

When backtracing circuits, I pull off the hot and neutral, then nut them to a resistor of known value, and then measure looking for that value of resistance.   I don't leave the neutral on the bus because I want to isolate/pair it.