Listen to these guys! "Sharing" neutrals can be tricky, especially if something goes wrong, there is an "open" neutral created, you need to troubleshoot something, or GFI's are involved. The simplest, surest, safest way is to run a separate neutral for each circuit- then make sure that the circuits stay separate!
Sometimes, if you know enough to ask a question, you already know you're in over your head. That's why even the best new journeyman recognises that his education is just beginning!
