Charging boat battery from two outboards at same time

Merc 135 and trolling Merc 9.9----I was cautioned when I bought the boat not to run both engines at same time, as the alternators charging could interfere with one another and possibly damage them .
any comments??

AEROMAN-grog said:

Merc 135 and trolling Merc 9.9----I was cautioned when I bought the boat not to run both engines at same time, as the alternators charging could interfere with one another and possibly damage them .
any comments??

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Not sure why they said that.
If one alternator has a higher output voltage, the other alternator should just shut down.
The alternator output diodes will prevent any reverse current from flowing to the low voltage device.
(See typical alternator schematic below