SOLAR SOLAR SOLAR SOLAR
No matter how many batteries you have consider solar charging to get them back up to full charge.
Two batteries is good though if you are camping in dark cloudy weather or in heavily forested areas where daily sunlight is sketchy.
Two CPAPs can tax one battery
Here in southern California a solar charger is much more important than the second battery
My Respironics system One CPAP draws about half of the current draw listed on the tag while not using the heated humidifier. It will use about 10% of a 100 amp hour battery
during the typical night.
The humidifier is just a resistance heater. The higher you turn it up the more current it uses. I have found that I can run my humidifier on 1 while on battery power and it does not blow the fuse, and it does not hit the battery too hard. Usually I just leave the heater off and fill the tank for passive humidification.