My first guess would be that when the current draw exceeds a certain threshold, (just a WAG here, say 60% of alternator output) the PCM will not allow the alternator to charge to prevent overheating/high wear issues on a single alternator. By going to dual alternators, you have effectively increased your charging capacity by 100%. Now, you've effectively cut the total current draw on your electrical system from the glow plugs by half. Since you'd now be at 30% of your total output, you would also now be below that hypothetical 60% threshold for the PCM to allow charging while the glow plugs are active.
No idea if the numbers or the theory or correct. Just hypothesizing...
Last edited by Davey~do; 11-10-2011 at 10:35 AM.
Reason: Spelling and english comprehension for teh win!