A multi-wire branch circuit (two hots from different phases sharing 1 neutral) is often found in the kitchen where it powers one receptacle which has the jumper connecting the upper and lower outets removed. The result is that you get two 15amps circuits at one receptacle. At the panel, both breakers should be bonded together so it is not possible to have one on and one off.

Code varies by region, but I do not think it is typically permitted in any other configuration.

There are also restrictions for having multiple circuits in a single junction box. Be careful working on this - even if the breaker is off, check for voltage with a non-contact tester to ensure there are no other live circuits.

A multi-wire branch circuit (two hots from different legs sharing 1 neutral) is often found in the kitchen where it powers one receptacle which has the jumper connecting the upper and lower outets removed. The result is that you get two 15amps circuits at one receptacle. At the panel, both breakers should be bonded together so it is not possible to have one on and one off.

Code varies by region, but I do not think it is typically permitted in any other configuration.

There are also restrictions for having multiple circuits in a single junction box. Be careful working on this - even if the breaker is off, check for voltage with a non-contact tester to ensure there are no other live circuits.