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If you turn off the front one after the rear drops, does the rear then come alive? If so, I suspect it's a voltage issue and can be fixed with an adjustment.
My 340 (similar setup to the parallel alternators in the 337s) will drop one alternator (usually the high one) if they differ by more than 0.5 volts. Both are working, but it won't parallel until the regulators are adjusted to have them match within 0.5 V. The good news is that it's a no-parts fix - just adjust the output voltage on each one to to match. I have to re-match them to compensate for drift about once a year.
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