There are several problems with this. One is the load on the neutral, another is the safety factor that the neutral carries current, and if it is disconnected it will most likely be live with voltage and can be a very serious shock hazard. Also, have two circuits using it you would have two sources for this to possibly occur.
You also CANNOT simply pull another wire for just the neutral. All the conductors of a circuit MUST be in the same conduit, cable or raceway.
Not to sound rude, but I truly think you need to read up a bit on residential wiring and get some of the basics down. All of your thoughts on this topic/job are incorrect and potentially very dangerous.