For the longest time, I wore rain pants as my second, windbreaker layer when it was cold out. But honestly, below a certain point, a thin windbreaker layer just doesn't cut it.
Eventually, I broke down and got a pair of lined coveralls. They were just as warm and durable as I'd hoped. However...
I had ordered a pair of jeans-style coveralls (pants, no bib) because I hated the whole "attempt to peel off an overcoat and who knows what in order to get the bib down to pee in a freezing porta-potty". This was a mistake. What actually happens is that without the bib straps to hold up the coveralls, the pants are a tremendous drag and need to be hitched up constantly if they're not so tight as to be painful. So I've traded 10 minutes of discomfort for an entire day of discomfort.
I'll figure out this field clothing thing eventually.