So I found the following circuit diagram in my home. After receiving a fun little shock, I found that someone was using 14/3 cabling to carry 2 different circuits to this 1 gang box (line on left, load on right). Since the load for Circuit 2 doesn't have a matching neutral, it's connected to Circuit 1's neutral line.
And before you ask, Circuit 1 and 2 are not MWBC circuits. Each one can be turned on/off independently. I'm presuming that this only works because the hot wires are on different phases, but this level of electrical work is a little over my head.
This doesn't seem to be very safe, though it hasn't caused a fire for (presumably) a number of years.
How do I go about fixing this so that it is safe?
I realize this can be and maybe should be fixed using a double pole breaker, which is certainly possible given they are neighbors in the breaker box, however I would like to keep them separate so I don't need to shut off power to half the house to work on an outlet (e.g. the circuits in the house are already on the large side of what I'd like).