You can either understand the concept of the neutral wire mathematically or practically. Since I'm more of a practical guy,let's take a look at the bigger picture. There is no neutral wire coming from the generator nor in transmission systems. The neutral wire is only implemented at the distribution (4-wire systems) and reticulation (live and neutral.... And earth) end of the picture.
Why is this you may wonder. The reason is that at the generator and transmission level, the lines or conductors have near identical impedance (ideally identical) therefore, the voltage between each of the 3 lines are of the same magnitude but 120 degrees apart from each other in phase. At the distribution level, your loads are far from identical, in fact each time a consumer of electricity switches the light on, the entire impedance of the distribution network changes.
This means that without a neutral wire, the voltage accross each load and the voltage between phases would be different, which is not ideal for both the consumer and the electrical system as it results in an imbalance of the electrical distribution system. Loads with greater impedance would require a larger volt drop across them than loads with less impedance.
The effects of this can be devastating on equipment not designed to handle the changing of the supply voltage, not to mention, your lights would fluctuate between dim and the sun like a disco club. This is where the neutral wire comes into play. The neutral wire is connected at a common point to all three phases. Ideally at $0\,V$ e.g star configuration.
This ensures that if there is a difference between each phases load impedance, that the voltage is kept constant. Which is why you only have $220\,V$(RMS) and $110\,V$ (RMS) or other standard voltage levels. It's the electrical current that should always be made capable of fluctuating. With the neutral implemented, we get constant voltage accross any load(impedance) with varying current.
How does the neutral wire make this possible? Since the neutral wire is a potential between all three phases, each phase along with the neutral wire can form an independent circuit e.g your house, hence live and neutral. It is the role of the neutral wire to carry any current as a result of the imbalance in impedance of each of the phases loads. This results in the maintenance of a stable standard voltage rating. Remember that voltage is relative to another voltage level.
If $220\,V$ is high, neutral is on the other hand is low, which also means that, since there is this potential difference, an electrical circuit may be formed in the first place.
Now, to answer the question posed in this topic, the live wire that can be traced back all the way to the nearest transformer(s) whose phase wires can be traced back to the generator's stator winding all the way at the power station. Neutral is the wire tied at the low potential end between each phase, enabling the completion of a circuit and maintaining a stable voltage level.
Since the neutral wire completes and electrical circuit (in terms of alternating current) it carries the same current as the live or phase wire tracing back to the generator, however, it's potential to earth is nearly $0\,V$. The voltage between the phase wire to earth would be $220\,V$, so the phase wire would alternate current direction between maximum positive and maximum negative peaks of the AC cycle.
Best Answer
Starting from the consumer unit / fuse box you have wires taking two different paths but ending up at the same place - a socket with a device attached.
So in effect you have the two live wires in parallel and the two neutral wires in parallel.
So if the current in one live wire is flowing from the consumer unit to device the same will be happening in the other wire.
When the current in one wire is reversed so the current will reverse in the other wire.
The magnitude of the currents flowing will follow the normal rules for parallel conductors.