Bill McKee writes:

I guess I better disconnect the 2nd factory battery in my Duramax Diesel
pickumup.

What you have is a compromised, but not uncommon, design. The biggest
batteries commonly sold aren't big enough for starting diesels. It would
be better if they put two 6V of the same size in series, or used a 24V
starter from two 12V in series, but it's cheapest to do it like you have.