I have a 2500HD Duramax in my truck. There are two batteries under
the hood. One is near the driver's side headlight and the other is
near the firewall on the passenger side.

I would like to know if the two batteries are directly in parallel or
if there is anything between them (besides battery cables).

My truck is 5 years old & I will be replacing the batteries within the
month. Is there any special way I should disconnect the batteries
when I replace them (since there are two)?

I've thought about replacing one with a new battery and then replacing
the other because I would always have 12V applied across the
electrical system. I wouldn't lose radio settings, etc.

I'd appreciate any suggestions.

They are in parallel directly connected.

You can do that. At this time I only have one in my vehicle and runs fine.
Though if it gets too cold, I may have to add the other one.

When you emoe the first positive cable wrap it in some newspaper or
cardboard, and tape it tight. If it hits ground you will be sorry.

Also be sure not to overtighten the bolts when you connect the cables.

Al