If you want to share a neutral, the ungrounded (hot) conductors must be on opposite phases. Someone needs to get a hold on this so called journeyman that you are working with! The situation that you are discribing is straight out of electric 101. When we did service up grades, we marked every conductor, and returned it to the phase that it belonged on. There were sometimes when we saw that someone else has moved some circuits around to phases that they should not have been on. In those cases, we would do some investigation, and take the steps needed to correct the problem.
Heck, Brian, send the guy to this forum. Maybe he'll learn something. I don't want to sound to harsh on the guy, but we're talking basic electricity and this guy is way out of line on it.
Cooling Down Now,