Someone correct me if I'm wrong, but the magnetic lines of flux issue only exists when current is flowing and it is not related to mixed voltages (at least at that level).
For example, even if the voltage level is the same, if one circuit is loaded, and the other is shut off at the supply, it is possible that the loaded circuit will induce a voltage in the wires in the other circuit.
As was stated, maybe not a good idea, but if the prospect of running a new conduit versus using an existing one is economically questionable, there is no NEC article to prohibit it.
I agree with Celtic. Not color coding the Neutrals is a good way to possibly hurt someone in the future.
[This message has been edited by Redsy (edited 03-08-2005).]
