single phase service
That shared neutral is doing twice the work of either of the ungrounded conductors.
If it's single phase (pair of 120Vs 180 degrees out of phase) electronic ballasts on both sides should be drawing their spikes of current at the same time (in respect to the timing of a 60 cycle waveform) and thus the neutral should be sharing/splitting the spikes the same way it would if the loads were just resistors. When you get neutral overload is when you have multiple phases, 120 degrees out of phase. Then the loads on phase A draw their spikes of current at a different time then those of phase B.
A bad connection on a neutral could cause problems like those you normally see with neutrals serving other kinds of loads.
[This message has been edited by wa2ise (edited 01-13-2006).]