Dual alternators, batteries went dead, whats the cause?

[H2OWALL]

My batteries went dead last night and I needed a jump in the middle of the snow storm. My lights were getting dimmer as I drove home. The battery light came on yesterday during the day but went off as I started driving.
Do I need to pull both alternators to get them checked, or is there a primary that could be causing this? Anything else that could be causing the problem?

 

[DarkShadowDiesel]

when my batteries started going, i replaced the primary (top) alternator and have been fine since.